In storage.connection.Connection.api_request, make response checking more robust

[dhermes]

From TODO:

    # TODO: Add better error handling.
      # TODO: Better checking on this header for JSON.

See storage.connection.Connection.api_request for relevant code and #146 for some context.

git blame output

b7f1b774 (JJ Geewax   2014-01-29 18:02:29 -0500 220)     # TODO: Add better error handling.
b7f1b774 (JJ Geewax   2014-01-29 18:02:29 -0500 227)       # TODO: Better checking on this header for JSON.

[tseaver]

The pyramid response/exception hierarchy might be useful as a start for making Connection.api_request raise more informative exceptions.

[tseaver]

See the JSON API docs for HTTP status codes