Handle empty deque on errorhandler lookup.

After registering a custom errorhandler by exception class, raising any
unhandled exception in a view function swallows the error and instead
throws an `IndexError` on trying to look up the appropriate handler.
This patch avoids the uninformative `IndexError` and preserves the
original exception by looping until the deque of classes is empty, not
forever.

flask/app.py

@@ -1421,7 +1421,7 @@ class Flask(_PackageBoundObject):
             # __mro__
             done = set()
 
-            while True:
+            while queue:
                 cls = queue.popleft()
                 if cls in done:
                     continue