Handle empty deque on errorhandler lookup.

[jmcarp]

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.

[jmcarp]

Code example:

from flask import Flask

app = Flask(__name__)

@app.errorhandler(TypeError)
def handle_typeerror(e):
    return 'got a typeerror!'

@app.route('/keyerror')
def keyerror():
    raise KeyError()

c = app.test_client()
c.get('/keyerror')

Since we've registered a handler for TypeError and not for KeyError, I would expect the KeyError to be thrown here. But instead we get an IndexError with no hint of the original error:

  File "/Users/jmcarp/code/flask/flask/app.py", line 1951, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/jmcarp/code/flask/flask/app.py", line 1605, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/jmcarp/code/flask/flask/app.py", line 1505, in handle_user_exception
    handler = self._find_error_handler(e)
  File "/Users/jmcarp/code/flask/flask/app.py", line 1440, in _find_error_handler
    .get(code))
  File "/Users/jmcarp/code/flask/flask/app.py", line 1425, in find_handler
    cls = queue.popleft()
IndexError: pop from an empty deque

With the proposed patch, the expected KeyError is thrown instead.

[untitaker]

Excellent, thanks!