show more verbose errors

[benben]

Right now, it is really hard to find out, why an api call is failing. For example when I try to create an entry with

content_type.entries.create(title: 'Test Title')
=> #<Contentful::Management::UnprocessableEntity: Validation error>

to really see what is actually not validating I just found this way:

e = content_type.entries.create(title: 'Test Title')
=> #<Contentful::Management::UnprocessableEntity: Validation error>
puts e.response.raw.body.to_s
=> <showing the full response body with the error>

Would be nice to have the error message(s) in an instance var or something like that in the exception.