Handle push errors to avoid broken sync

[wonder-sk]

Sometimes it may happen that pushing a new version of a project fails for some reason (e.g. a network issue or a temporary server problem). Currently if that happens, the local directory is left with modifications - so the next time the daemon wakes up to do pull+push, the pull stage does not like the fact there are some pending changes, and the whole sync breaks.

If push fails, we should clean up the local directory - there's already a to-do left in the code:
https://github.com/lutraconsulting/mergin-db-sync/blob/master/dbsync.py#L486

[Koivusto]

We've faced this issue as well. Would be great to have more tools to tackle this!

[HugoP27]

We also face this issue

[wonder-sk]

Fixed in #83 - if there are any leftovers in the working directory, they will be cleaned up.