should throw error on bad request or no markup

Backstop should throw an error if I point to a non-existent endpoint or I receive an unexpected http status or if I otherwise receive no markup.

The issue names no files, tests, or entry points, so first locate the request and markup-handling path in BackstopJS. Reproduce a non-existent endpoint, an unexpected HTTP status, and a response with no markup; done means each case throws an error rather than continuing silently.

Written by the indexing model from the issue text.