Warn if a backup fails after old backups were removed

With --remove-before-backup, a failed backup leaves only the kept
backups, or none with keep=0. Log an error and send a notification
in that case, and document it in the man page.

simple_backup/simple_backup.py

@@ -160,6 +160,7 @@ class Backup:
         self._ssh = None
         self._password_auth = False
         self._password = None
+        self._removed_count = 0
 
     def check_params(self, homedir: str = '') -> int:
         """Check if parameters for the backup are valid"""
@@ -284,6 +285,8 @@ class Backup:
                         logger.error(
                             'Error while removing backup %s. Permission denied', dirs[i])
 
+        self._removed_count = count
+
         if count == 1:
             logger.info('Removed %d backup', count)
         elif count > 1:
@@ -602,6 +605,17 @@ class Backup:
                 else:
                     logger.error(output)
 
+        if self._removed_count > 0 and returncode not in [0, 24]:
+            # With --remove-before-backup, old backups are already gone if the new one failed
+            if self.keep == 0:
+                message = 'All old backups were removed before the backup, and the new backup may be ' +\
+                    'incomplete. No complete backup is available'
+            else:
+                message = 'Old backups were removed before the backup, and the new backup may be incomplete'
+
+            logger.error(message)
+            _notify(message)
+
         if self.keep != -1 and not self._remove_before:
             # Don't delete old backups if the new one may be incomplete
             if returncode in [0, 24]: