Fix parsing of dangling dep check

Buildbot parses failure messages that look like ninja errors, but this
error doesn't look like a ninja error. We used to output this via
stderr, which buildbot would fall back to, but that had synchronization
issues between stderr and stdout.

So switch this over to use the status API, marking this action as failed
when necessary, which will display a ninja-like error that buildbot can
parse.

Bug: 112007097
Test: add dangling dep, look at output.
Change-Id: Ib2bbba4373ea22ac01af8aa6bbf295296a93e94d