test_harness.h: Explain failure on terminal signal

The chroot and consume* changes introduce a crasher in the unittests.
This adds proper error messaging on that case.

BUG=none
TEST=FEAUTRES=test emerge-x86-alex chromeos-minijail (with pending change)
     see chroot crasher emitted warning.

Change-Id: I2a312b961a321e3bf55645aed9c6f6480dd958c9
Reviewed-on: https://gerrit.chromium.org/gerrit/10539
Commit-Ready: Will Drewry <[email protected]>
Reviewed-by: Will Drewry <[email protected]>
Tested-by: Will Drewry <[email protected]>
1 file changed