Handle sync conflicts more gracefully

[ChristophWurst]

Expected behavior

#2064 correctly catches concurrent sync attempts of the same mailbox. Running into this is expected and not an error.

Actual behavior

This gets logged to nextcloud.log, sent to Sentry and might pop up as error for the user.

Mail app

Mail app version:

One with #2064.

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Solution: I think we could just ignore the request. Or send again later. We just have to be careful not to send too many requests. Exponential back-off could help as well.