zed: Print return code on failed zpool_prepare_disk

[tonyhutter]

Motivation and Context

Better zed error logging

Description

We had a case where we were autoreplacing a disk and zpool_prepare_disk failed for some reason, and ZED didn't log the return code. This commit logs the code.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

Seeing that in some cases we return errno codes there I was thinking if we could report strings there, as done in some other messages. But at least libzfs_run_process_get_stdout() seem to return all kinds of a mess.