Add fall back message for custom error messages

[m4tthumphrey]

Github also throws custom error messages and the switch doesn't catch it.

[errors] => Array
(
    [0] => Array
    (
        [code] => custom
        [resource] => Repository
        [field] => name
        [message] => name can't be private. You are over your quota.
    )
)

Adding a default fixes this.

[stloyd]

Are you sure that error is thrown with code 422 ? Because only in that case it would be catched... Also an PR do GH API docs would be awesome =)

[m4tthumphrey]

I only found it because I received the following as an exception message:

Validation failed: 

Ie no message..

Why do I need to touch the docs for this?

[stloyd]

Merged then! Thanks!

About PR for GH API docs, it would allow other people to don't miss that there are also such errors, look that only those what I have implemented are listed here: http://developer.github.com/v3/#client-errors