Explain why BuildImage failed.

BuildImage can return false for lots of reasons. Log two of the more
plausible ones. You'll still need to edit this file and pass True to
RunCommand to work out *what* went wrong, but at least this logging will
tell you *where* things went wrong.

Bug: http://b/63142920
Test: debugged my actual problem
Change-Id: If84427058108dcac0a1b5e2216b8bb11eef522ca