Acccept any HTTP 2xx status as success in Harbormaster

Summary: Ref T9478. This should probably be configurable eventually, but for now treat any 200-block status as success. Also show the result code.

Test Plan:

• Hit a bad URI, saw "HTTP 503" + failure.
• Hit a good URI, saw "HTTP 200" + success.

Reviewers: chad, hach-que

Reviewed By: chad, hach-que

Maniphest Tasks: T9478

Differential Revision: https://secure.phabricator.com/D14226