chore: JWT warnings on failed JWT ops

I'd like to have a warning meggase on failing JWT validations, or may use a debug for that and not a warning?

Author: peppelinux

src/cryptojwt/jwt.py

@@ -372,11 +372,12 @@ def unpack(self, token):
         if self.msg_cls:
             _msg_cls = self.msg_cls
         else:
-            try:
-                # try to find a issuer specific message class
-                _msg_cls = self.iss2msg_cls[_info["iss"]]
-            except KeyError:
-                _msg_cls = None
+            _msg_cls = self.iss2msg_cls.get(_info["iss"], None)
+            if not _msg_cls:
+                LOGGER.warning(
+                    "Exception unpacking a received JWT "
+                    f"for the issuer {_info['iss']}"
+                )
 
         if _msg_cls:
             vp_args = {"skew": self.skew}