kunit: tool: tweak error message when no KTAP found

We currently tell people we "couldn't find any KTAP output" with no
indication as to what this might mean.

After this patch, we get:

$ ./tools/testing/kunit/kunit.py parse /dev/null
============================================================
[ERROR] Test: <missing>: Could not find any KTAP output. Did any KUnit tests run?
============================================================
Testing complete. Ran 0 tests: errors: 1

Note: we could try and generate a more verbose message like
> Please check .kunit/test.log to see the raw kernel output.
or the like, but we'd need to know what the build dir was to know where
test.log actually lives.

This patch tries to make a more minimal improvement.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>