Bugfix: Return 500 "Internal server error" if files cannot be read

When files in a repository cannot be read by rest-server, for example after
running `restic prune` directly on the server hosting the repositories in a
way that causes filesystem permissions to be wrong, rest-server previously
returned 404 "Not Found" as status code. This was causing confusing for users.

The error handling has now been fixed to only return 404 "Not Found" if the
file actually does not exist. Otherwise a 500 "Internal server error" is
reported to the client and the underlying error is logged at the server side.

https://github.com/restic/restic/issues/1871
https://github.com/restic/rest-server/pull/195