dfa: Report a more reasonable error if an automaton cannot be canonised

lib/dfa.py

@@ -330,6 +330,11 @@ class DFA:
                 if tgt is not None and tgt not in state_map.keys():
                     queue.append(tgt)
 
+        for st, _ in self.transition.keys():
+            if st not in state_map:
+                raise Exception("Automaton cannot be canonised, it has "
+                                "unreachable states")
+
         return DFA(set(state_map.values()),
                    deepcopy(self.characters),
                    {(state_map[st], ch): state_map[tgt]