Handle non-existing backup file (#5590)

* Make the API return 404 for non-existing backup files

* Introduce BackupFileNotFoundError exception

* Return 404 on full restore as well

* Fix remaining API tests

* Improve error handling in delete

* Fix pytest

* Fix tests and change error handling to agreed logic

---------

Co-authored-by: Mike Degatano <michael.degatano@gmail.com>

supervisor/exceptions.py

@@ -659,6 +659,10 @@ class BackupJobError(BackupError, JobException):
     """Raise on Backup job error."""
 
 
+class BackupFileNotFoundError(BackupError):
+    """Raise if the backup file hasn't been found."""
+
+
 # Security