Write to STDERR instead of php://stderr

Summary:
It doesn't print anything under HPHP.
I've reported it but in the meantime...

Also the semantics of file_put_contents('php://stderr') is somewhat weird.
It suggests that the whole error channel is overwritten by the message which is not the case.

Test Plan:

$ bin/storage --dry-run

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1261

Differential Revision: https://secure.phabricator.com/D3661