Handle OS errors on backup create (#5662)

* Handle permission error on backup create

Make sure we handle (write) permission errors when creating a backup.

* Introduce BackupFileExistError and BackupPermissionError exceptions

* Make error messages a bit more uniform

* Drop use of exclusive mode

SecureTar does not handle exclusive mode nicely. Drop use of it for now.

supervisor/exceptions.py

@@ -663,6 +663,14 @@ class BackupFileNotFoundError(BackupError):
     """Raise if the backup file hasn't been found."""
 
 
+class BackupPermissionError(BackupError):
+    """Raise if we could not write the backup due to permission error."""
+
+
+class BackupFileExistError(BackupError):
+    """Raise if the backup file already exists."""
+
+
 # Security