Bugfix: Return "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, then
rest-server returned "Not Found" as status code. This is extremely confusing
for users.

The error handling has been fixed to only return "Not Found" if the file
actually does not exist. Otherwise an internal server error is reported to the
user and the underlying error is logged at the server side.

https://github.com/restic/restic/issues/1871
https://github.com/restic/rest-server/pull/194