Work around Gmail "Some messages could not be FETCHed" errors

[rgrove]

When Gmail returns a "Some messages could not be FETCHed" error in response to a batch fetch operation, Larch should retry the fetch, gradually decreasing the batch size each time until it succeeds and then marking and ignoring the corrupt message.

See http://groups.google.com/group/larch/msg/6e045bba542fbde2

[efgonzalez]

Any progress here? Perhaps I can help implementing this.

[rgrove]

I had been intending to address this in the refactor branch, but it's not yet complete. What you see there is what I've got so far.

The amount of time I'm able to devote to Larch is virtually nil these days, so help would be more than welcome.