Report error when datasources.json does not parse

[chrisknoll]

When there is a malformed JSON returned from the AJAX request to get the datasources, if the file is loaded successfully you get a 200 status, but the done() promise doesn't fire because the response could not be parsed as JSON. .fail() will trigger, so we should add the fail() handler to this call and report an error if there is a failure. Otherwise, the app seems to hang on 'loading' without reporting any error.