kunit: show error if kunit results are not present

Currently, if the kernel is configured incorrectly or if it crashes before any
kunit tests are run, kunit finishes without error, reporting
that 0 test cases were run.

To fix this, an error is shown when the tap header is not found, which
indicates that kunit was not able to run at all.

Signed-off-by: Uriel Guajardo <urielguajardo@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>