Explicitly signal incorrect 'remote' metadata conf.

Author: Rebecka Gulliksson

src/saml2/mdstore.py

@@ -847,6 +847,8 @@ def load(self, typ, *args, **kwargs):
             kwargs.update(_args)
             _md = InMemoryMetaData(self.attrc, args[0])
         elif typ == "remote":
+            if "url" not in kwargs:
+                raise ValueError("Remote metadata must be structured as a dict containing the key 'url'")
             key = kwargs["url"]
             for _key in ["node_name", "check_validity"]:
                 try: