backups: Handle error when there is not enough space on disk

Tests:

- Fill up disk to have very little space left. Perform a large backup operation
by writing a large file to one of the directories to be backed up. Try to
perform the backup and notice that a nice error message is shown that there is
not enough disk space instead of a generic error message.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/backups/errors.py

@@ -33,3 +33,7 @@ class BorgArchiveDoesNotExist(BorgError):
 
 class BorgBusy(BorgError):
     """Borg could not acquire lock being busy with another operation."""
+
+
+class BorgNoSpace(BorgError):
+    """There is not enough space left on the device to perform operation."""

plinth/modules/backups/privileged.py

@@ -87,6 +87,11 @@ KNOWN_ERRORS = [
         'message': _('Backup system is busy with another operation.'),
         'raise_as': errors.BorgBusy,
     },
+    {
+        'errors': ['No space left on device'],
+        'message': _('Not enough space left on the disk or remote location.'),
+        'raise_as': errors.BorgNoSpace,
+    },
 ]