Changed downloading routine to raise an Error in case of HTTP errors.
Changed downloading routine to raise an Error in case of HTTP errors.

urllib.request.retrieve does this in Python3, but in Python2
urllib.retrieve would only return the error page as the requested file.
Using urlopen, any HTTP error should result in an error being raised in
the Python code for any Python version.

NeuralEnsemble/python-neo,rgerkin/python-neo,INM-6/python-neo,samuelgarcia/python-neo,JuliaSprenger/python-neo,apdavison/python-neo