Refactor UI auth implementation

Instead of returning False when auth is incomplete, throw an exception which
can be caught with a wrapper.

synapse/api/errors.py

@@ -140,6 +140,22 @@ class RegistrationError(SynapseError):
     pass
 
 
+class InteractiveAuthIncompleteError(Exception):
+    """An error raised when UI auth is not yet complete
+
+    (This indicates we should return a 401 with 'result' as the body)
+
+    Attributes:
+        result (dict): the server response to the request, which should be
+            passed back to the client
+    """
+    def __init__(self, result):
+        super(InteractiveAuthIncompleteError, self).__init__(
+            "Interactive auth not yet complete",
+        )
+        self.result = result
+
+
 class UnrecognizedRequestError(SynapseError):
     """An error indicating we don't understand the request you're trying to make"""
     def __init__(self, *args, **kwargs):