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: NUriel Guajardo <urielguajardo@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>