The HTTP error handler in GitHub was assuming that error classes always

had an HTTP status code, but this is only the case for HTTPError and

not URLError. If an error occurs due to an outage or something other

than an HTTP response, we'd crash.
This fixes the code to provide an HTTP status code only if it exists on

the error object.

Turned off wifi and attempted to load a diff locally. Saw an error about

issues accessing the file, rather than a crash traceback.