Improve error handling in `check`

[simonsan]

First, the most important thing is that prune (being the only command which removes pack or index files) doesn't leave the repository in an unsane condition. This is why check is called here. (to admit, check currently does not produce an error but only logs an error! - I'm about to improve this next).

Originally posted by @aawsome in https://github.com/rustic-rs/rustic_core/pull/221#discussion_r1590161138