Trap internal AssertionError from python libraries

For some really broken messages, we end up in a cannot-happen codepath.
Trap this one and just consider that MIME part empty, and try again
later.

In passing, also change it so we continue loading after failures of parsing.
We continued in the mode where we just generated diffs, but not when
making updates. Now continue in both cases, but of course don't do the
actual update if the parsing failed.

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