Remove special UTF-8 decoding errors handling for the "message" value
of a swh revision dictionary and use global decoding errors handler
instead.
As a reminder, the global handler for UTF-8 decoding errors performs
the following actions:
- It puts all key names of a dictionary where UTF-8 decoding of values failed in a list and store it under a new key "decoding_failures".
- A string that could not be decoded will have the bytes of its invalid UTF-8 sequences escaped.
Also add section about UTF-8 decoding errors in top level API documentation.
Closes T2617