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

[nathanperkins]

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.