diff --git a/dulwich/errors.py b/dulwich/errors.py
index 54022b05..9a7a8727 100644
--- a/dulwich/errors.py
+++ b/dulwich/errors.py
@@ -1,199 +1,204 @@
 # errors.py -- errors for dulwich
 # Copyright (C) 2007 James Westby <jw+debian@jameswestby.net>
 # Copyright (C) 2009-2012 Jelmer Vernooij <jelmer@jelmer.uk>
 #
 # Dulwich is dual-licensed under the Apache License, Version 2.0 and the GNU
 # General Public License as public by the Free Software Foundation; version 2.0
 # or (at your option) any later version. You can redistribute it and/or
 # modify it under the terms of either of these two licenses.
 #
 # Unless required by applicable law or agreed to in writing, software
 # distributed under the License is distributed on an "AS IS" BASIS,
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
 # You should have received a copy of the licenses; if not, see
 # <http://www.gnu.org/licenses/> for a copy of the GNU General Public License
 # and <http://www.apache.org/licenses/LICENSE-2.0> for a copy of the Apache
 # License, Version 2.0.
 #
 
 """Dulwich-related exception classes and utility functions."""
 
+
+# Please do not add more errors here, but instead add them close to the code
+# that raises the error.
+
+
 import binascii
 
 
 class ChecksumMismatch(Exception):
     """A checksum didn't match the expected contents."""
 
     def __init__(self, expected, got, extra=None):
         if len(expected) == 20:
             expected = binascii.hexlify(expected)
         if len(got) == 20:
             got = binascii.hexlify(got)
         self.expected = expected
         self.got = got
         self.extra = extra
         if self.extra is None:
             Exception.__init__(
                 self,
                 "Checksum mismatch: Expected %s, got %s" % (expected, got),
             )
         else:
             Exception.__init__(
                 self,
                 "Checksum mismatch: Expected %s, got %s; %s" % (expected, got, extra),
             )
 
 
 class WrongObjectException(Exception):
     """Baseclass for all the _ is not a _ exceptions on objects.
 
     Do not instantiate directly.
 
     Subclasses should define a type_name attribute that indicates what
     was expected if they were raised.
     """
 
     def __init__(self, sha, *args, **kwargs):
         Exception.__init__(self, "%s is not a %s" % (sha, self.type_name))
 
 
 class NotCommitError(WrongObjectException):
     """Indicates that the sha requested does not point to a commit."""
 
     type_name = "commit"
 
 
 class NotTreeError(WrongObjectException):
     """Indicates that the sha requested does not point to a tree."""
 
     type_name = "tree"
 
 
 class NotTagError(WrongObjectException):
     """Indicates that the sha requested does not point to a tag."""
 
     type_name = "tag"
 
 
 class NotBlobError(WrongObjectException):
     """Indicates that the sha requested does not point to a blob."""
 
     type_name = "blob"
 
 
 class MissingCommitError(Exception):
     """Indicates that a commit was not found in the repository"""
 
     def __init__(self, sha, *args, **kwargs):
         self.sha = sha
         Exception.__init__(self, "%s is not in the revision store" % sha)
 
 
 class ObjectMissing(Exception):
     """Indicates that a requested object is missing."""
 
     def __init__(self, sha, *args, **kwargs):
         Exception.__init__(self, "%s is not in the pack" % sha)
 
 
 class ApplyDeltaError(Exception):
     """Indicates that applying a delta failed."""
 
     def __init__(self, *args, **kwargs):
         Exception.__init__(self, *args, **kwargs)
 
 
 class NotGitRepository(Exception):
     """Indicates that no Git repository was found."""
 
     def __init__(self, *args, **kwargs):
         Exception.__init__(self, *args, **kwargs)
 
 
 class GitProtocolError(Exception):
     """Git protocol exception."""
 
     def __init__(self, *args, **kwargs):
         Exception.__init__(self, *args, **kwargs)
 
     def __eq__(self, other):
         return isinstance(self, type(other)) and self.args == other.args
 
 
 class SendPackError(GitProtocolError):
     """An error occurred during send_pack."""
 
 
 # N.B.: UpdateRefsError is no longer used and will be result in
 # Dulwich 0.21.
 # remove: >= 0.21
 class UpdateRefsError(GitProtocolError):
     """The server reported errors updating refs."""
 
     def __init__(self, *args, **kwargs):
         self.ref_status = kwargs.pop("ref_status")
         super(UpdateRefsError, self).__init__(*args, **kwargs)
 
 
 class HangupException(GitProtocolError):
     """Hangup exception."""
 
     def __init__(self, stderr_lines=None):
         if stderr_lines:
             super(HangupException, self).__init__(
                 "\n".join(
                     [line.decode("utf-8", "surrogateescape") for line in stderr_lines]
                 )
             )
         else:
             super(HangupException, self).__init__(
                 "The remote server unexpectedly closed the connection."
             )
         self.stderr_lines = stderr_lines
 
     def __eq__(self, other):
         return isinstance(self, type(other)) and self.stderr_lines == other.stderr_lines
 
 
 class UnexpectedCommandError(GitProtocolError):
     """Unexpected command received in a proto line."""
 
     def __init__(self, command):
         if command is None:
             command = "flush-pkt"
         else:
             command = "command %s" % command
         super(UnexpectedCommandError, self).__init__(
             "Protocol got unexpected %s" % command
         )
 
 
 class FileFormatException(Exception):
     """Base class for exceptions relating to reading git file formats."""
 
 
 class PackedRefsException(FileFormatException):
     """Indicates an error parsing a packed-refs file."""
 
 
 class ObjectFormatException(FileFormatException):
     """Indicates an error parsing an object."""
 
 
 class NoIndexPresent(Exception):
     """No index is present."""
 
 
 class CommitError(Exception):
     """An error occurred while performing a commit."""
 
 
 class RefFormatError(Exception):
     """Indicates an invalid ref name."""
 
 
 class HookError(Exception):
     """An error occurred while executing a hook."""