handle config errors

This is not a full solution, we should actually be restarting the builds on server_error, but at least we tell the user what's going on and we don't run with the standard config on server error anymore.

Also displays a message (but still runs the build) if there simply is no .travis.yml.

`.result` is set by [fetch_config](https://github.com/travis-ci/travis-core/blob/master/lib/travis/github/services/fetch_config.rb#L13-L23).