Catch fatal unpacking error
[mms] Fix resetting capability list when internally cached authentication information becomes invalid.
fix variable
Avoid warnings
Fix for a certain use case
phpdoc
Don't sending thumbnail data with message data by default

While good in theory, it breaks down once multiple images per message
are present.  Then thumbnail generation becomes blocking for viewing the
message at all, and it can take many seconds before the message is
loaded.

Better solution is to revert to previous method of using separate URL
access to load each thumbnail.