buildbucket_lib currently assumes that a return code of 200 means that the API request was successful.

This is not true (see  crbug.com/720119 ).

There are currently cases where we ignore error responses from the API (such as when scheduling builds). There are other places where the users of buildbot_lib examine the API response to see if there was an error.

To be more straight forward and more consistent, we should raise an exception whenever we get a 200 response code, and the json response contains an 'error' key.

We can attach the details of that json response to the exception for clients that need it.