Delete empty bugreport files onError

Bug: 152839953
Test: Take another bugreport, while one is in progress.
Without this change: bugreport files for the second bugreport can be
seen in /bugreports
With this change: no file for the second bugreport can be seen. This
is the expected behavior.

Change-Id: I45c1f66f3a52ba9a31413cf9748bc5504eaa65d0