add KeyNotFound IssuerNotFound

Author: jschlyter

src/cryptojwt/exception.py

@@ -43,8 +43,12 @@ class MissingKey(JWKESTException):
     """No usable key"""
 
 
-class MissingIssuer(JWKESTException):
-    """No usable issuer"""
+class KeyNotFound(KeyError):
+    """Key not found"""
+
+
+class IssuerNotFound(KeyError):
+    """Issuer not found"""
 
 
 class KeyIOError(Exception):