raise Exception when issuer is not found

Author: jschlyter

src/cryptojwt/key_jar.py

@@ -467,7 +467,7 @@ def _add_key(self, keys, issuer_id, use, key_type='', kid='',
         _issuer = self._get_issuer(issuer_id)
         if _issuer is None:
             logger.error('Issuer "{}" not in keyjar'.format(issuer_id))
-            return keys
+            raise IssuerNotFound(issuer_id)
 
         logger.debug('Key summary for {}: {}'.format(issuer_id, _issuer.key_summary()))