LocalProtocolError raised in start_next_cycle shows states

This makes the error much more informative, allowing the reason for it
to be more readily deduced.

Author: pgjones

h11/_state.py

@@ -307,7 +307,9 @@ def _fire_state_triggered_transitions(self):
 
     def start_next_cycle(self):
         if self.states != {CLIENT: DONE, SERVER: DONE}:
-            raise LocalProtocolError("not in a reusable state")
+            raise LocalProtocolError(
+                "not in a reusable state. self.states={}".format(self.states)
+            )
         # Can't reach DONE/DONE with any of these active, but still, let's be
         # sure.
         assert self.keep_alive