autotest: improve test_that.py error message when no tests found

BUG=chromium:419931
TEST=None

Change-Id: Ic3b71c7a23183fc89436a757064dfb7852a9c34f
Reviewed-on: https://chromium-review.googlesource.com/221315
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
1 file changed