Introduce a way to distinguish different github errors

[mikegirkin]

I discovered that there is not way to distinguish between different type of errors in the application using github4s.
This small PR introduces another type for the errors, so the consumer get more information to deal with:

• ability to distinguish between HTTP errors (ie. file not found) and other
• ability to deal with HTTP errors differently based on status code

[BenFradet]

The problem is that I think this just ends up being a rename of UnexpectedException if I'm not mistaken?

[mikegirkin]

@BenFradet
Not exactly, I've added http code to be able to distinguish on the client between different situations with a valid response from github received.
UnexpectedException is used also in JS-specific part to recover from Future errors and save the message.

[BenFradet]

Ah yes forgot about those damned Futures, wdyt about renaming it to UnsuccessfulHttpRequest, Github being implied since it subclasses GHException?

[mikegirkin]

@BenFradet Agree, I'll do that

[BenFradet]

you need to rename the other 2 occurrences 👍

[mikegirkin]

@BenFradet Silly mistake. Apologies.

[BenFradet]

Ran the tests successfully 👍

[juanpedromoreno]

Thanks @mikegirkin !