Change: Return exit code 10 and 11 for non-existing and locked repository

If a repository does not exist or cannot be locked, restic previously always
returned exit code 1. This made it difficult to distinguish these cases from
other errors.

Restic now returns exit code 10 if the repository does not exist, and exit code
11 if the repository could be not locked due to a conflicting lock.

https://github.com/restic/restic/issues/956
https://github.com/restic/restic/pull/4884
