Handle 'status:fail' responses from PhantomJS land

When a server is not running at the specified host:port or
if there is a DNS failure, PhantomJS will respond with
status=fail, no response code, and a minimal HTML document.
We should not return that document, but throw an exception instead.
Merge pull request #473 from dholdren/master

Handle 'status: fail' responses from PhantomJS land instead of returning minimal HTML (i.e. "blank page")