Detect and provide better error message when pre-user-code execute-side dependencies are missing

[JoshKarpel]

In particular, cloudpickle. If cloudpickle is missing execute-side, we error out before managing to do any error handling and don't provide a useful error message to the user. The component is simply held because the exit code wasn't zero, and HTMap doesn't hint the user to look in the component stdout/stderr to see what's wrong because it assumes the job was held for "unrelated" reasons.