Bugfix: Reply "insufficient storage" on disk full or over-quota

When there was no space left on disk, or any other write-related error,
rest-server was replying with HTTP status code 400 (Bad request). This is
misleading (restic client will dump the status code to the user).

This has been fixed changing the behaviour so two different statuses are used:
* HTTP 507 "Insufficient storage" is the status on disk full or repository
  over-quota
* HTTP 500 "Internal server error" on other disk-related errors

https://github.com/restic/rest-server/issues/155
https://github.com/restic/rest-server/pull/160