skaffold verify should exit with non-zero code when aborted by signal

When a run of `skaffold verify` is aborted by signal, it returns an exit code of `0`, indicating success.

This is problematic when using `skaffold verify` for testing in CI, because it will indicate that tests passed, when they did not.

`skaffold verify` should exit with a non-zero exit code when aborted by signal to indicate that tests did not pass.