common/converters: Harmonize UTF-8 decoding errors handling

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.

Related to T2617