Refactor HTTP request handling in `SproxydFileSystem`

In several methods of the `SproxydFileSystem` class, a similar pattern
of performing an HTTP request and handling the response was used. This
commit abstracts out this common pattern, which simplifies code and
should ease unit-testing.
Have you considered the inspect module ? We could inspect the stack on `unexpected_http_status`. I am not sure I'd like to use inspection here but that's an option to reduce the  number of parameters.