Ref T13258. Make the "Affects Buildable" option actually work.

• As in previous change, created a "wait for HTTP request" build plan and had it always run against every revision.
• Created revisions, waited a bit, then sent the build a "Fail" message, with different values of "Affects Buildable":
• "Always": Same behavior as today. Buildable waited for the build, then failed when it failed.
• "While Building": Buildable waited for the build, but passed even though it failed (buildable has green checkmark even though build is red):

• "Never": Buildable passed immediately (buildable has green checkmark even though build is still running):