It's missing an `)`

Change the links of the logos of different browsers according to the original repository.

Fixes #615

I asked the question in #516, pretty sure it will help other people too.

* Adding proxy auth

Docs were suggesting that accessing the response with `catch` was the same as accessing it with `then`. Split the two cases and point to the Handling Errors section. Also mention the case where a rejection callback is specified as second parameter of `then`.

Adding support for custom HTTP status code error ranges