verifier-cli: get_chunk ignores failures

[flihp]

If we call get_chunk w/ an invalid size (more data than the task / op wants) the function will currently ignore the failure. This is because we only check the humility hiffy return code and it's 0 in this case. We need to parse stdout to detect the following string:

<task>.<op>() => Err(<Complex error: {error-type}>)