Check if the code is a proper integer before comparison

Boto's [JSONReponseError](http://boto.readthedocs.org/en/latest/ref/boto.html#boto.exception.JSONResponseError) provides an attribute `code`, which makes Flask Restful break in the event that is the exception. This adds some very basic type checking to make sure that the real error isn't hidden.
```
Traceback (most recent call last):
  File "/Users/newmaniese/VirtualEnvs/bobcat2/lib/python3.4/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/newmaniese/VirtualEnvs/bobcat2/lib/python3.4/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/Users/newmaniese/VirtualEnvs/bobcat2/src/flaskrestful/flask_restful/__init__.py", line 261, in error_router
    return self.handle_error(e)
  File "/Users/newmaniese/VirtualEnvs/bobcat2/lib/python3.4/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/newmaniese/VirtualEnvs/bobcat2/lib/python3.4/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/newmaniese/VirtualEnvs/bobcat2/src/flaskrestful/flask_restful/__init__.py", line 261, in error_router
    return self.handle_error(e)
  File "/Users/newmaniese/VirtualEnvs/bobcat2/src/flaskrestful/flask_restful/__init__.py", line 284, in handle_error
    if code >= 500:
TypeError: unorderable types: str() >= int()
```