kunit: tool: print clearer error message when there's no TAP output

Before:
$ ./tools/testing/kunit/kunit.py parse /dev/null
...
[ERROR] Test : invalid KTAP input!

After:
$ ./tools/testing/kunit/kunit.py parse /dev/null
...
[ERROR] Test <missing>: could not find any KTAP output!

This error message gets printed out when extract_tap_output() yielded no
lines. So while it could be because of malformed KTAP output from KUnit,
it could also be due to not having any KTAP output at all.

Try and make the error message here more clear.

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