Error reporting

[hannesm]

While working with @reynir, we encountered that we'd like to preserve some errors, esp. in set and change_and_push:

• There may be a network error (remote host not reachable / available) -- retrying in a bit may solve the issue
• Also, the remote git server may have a newer head than us, and the sync.push may fail (unless it is a force-push, if so, we should specify that [unless it is already stated in the documentation])
• In change_and_push, if locally the t evolves, we currently error with a message -- here a rebase/merge would be the best, but a "local modification in a different task" would be welcome as well.

What do you think? At the moment, there are several constructors in write_error, but not of the kind network / remote has a newer version. We also didn't quite understand what to do with `Hash_not_found and `Reference_not_found -- our focus was on "what does a API client want to do, and figure out when a retry make sense".