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

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

rest-server now replies with two different status codes in these situations:
* HTTP 507 "Insufficient storage" is the status on disk full or repository
  over-quota
* HTTP 500 "Internal server error" is used for other disk-related errors

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