DateTimeImmutable::createFromFormat returns false on failure

Summary: Fixes #3224

I want falseable types...

Closes https://github.com/facebook/hhvm/pull/4271

Reviewed By: @paulbiss

Differential Revision: D1688002

Signature: t1:1688002:1416500930:ed9ca7ec2b9aa730b36bbd6f45f9379986368d90