Better handling for JSON response errors without __type

Fixes #235. Adds handling for `x-amzn-errortype` headers as returned by
Elastic Transcoder. AWS CLI now prints out the exception name along with
any message from the body.

Updated JSON response tests to support a special `__headers` key to set
the response headers. Modified an existing test to ensure the header
gets parsed properly. Retry tests cover everything we are using already,
so I left those alone.

@jamesls please review