Add ability to enable raise_errors faraday middleware

Useful for checking if connection to Digital Ocean is 401'd because your credentials are expired or incorrect

I was going to put it in as default, but I thought it might be better to make it a choice (as you might not like the full-stack trace everytime you get a 404 or something)

Will help fix https://github.com/pearkes/tugboat/issues/53

@rmoriz any thoughts?