Add exception for bad custom Wikifactory API URL

Author: penyuan

oshminer/errors/exceptions.py

@@ -4,4 +4,7 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 class BadRepoError(Exception): 
+    pass
+
+class BadWIFAPIError(Exception):
     pass