Fallthrough to original error handler when occur unhandled exception

[Sheile]

Problem

According to comment in code, Api#error_router should propagate exception which can't handle in flask-restplus to next error handler.
But, current implementation in handle_error suppress unhandled exception and break hierarchical error handling.

As the result, The original Flask error handler for the app, or other following error handler is NOT called.

Fixed by

Change the handle_error to propagate unhandled exception to original handler.

Reference: Api#error_router

def error_router(self, original_handler, e):
    '''
    This function decides whether the error occured in a flask-restplus
    endpoint or not. If it happened in a flask-restplus endpoint, our
    handler will be dispatched. If it happened in an unrelated view, the
    app's original error handler will be dispatched.
    In the event that the error occurred in a flask-restplus endpoint but
    the local handler can't resolve the situation, the router will fall
    back onto the original_handler as last resort.

    :param function original_handler: the original Flask error handler for the app
    :param Exception e: the exception raised while handling the request
    '''
    if self._has_fr_route():
        try:
            return self.handle_error(e)
        except Exception:
            pass  # Fall through to original handler
    return original_handler(e)

[coveralls]

Coverage decreased (-0.04%) to 96.804% when pulling b67e315 on Sheile:fix/fallthrough-to-original-handler into a8f3582 on noirbizarre:master.