add MissingIssuer exception

Author: jschlyter

src/cryptojwt/exception.py

@@ -43,6 +43,10 @@ class MissingKey(JWKESTException):
     """ No usable key """
 
 
+class MissingIssuer(JWKESTException):
+    """No usable issuer"""
+
+
 class KeyIOError(Exception):
     pass