Change: return exit code 10 or 11 if repository does not exist or is locked

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

Now, restic 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