core: Add information on which condition failed to job skipped format string

core: Add information on which condition failed to job skipped format string

When a job is skipped, it's useful to know exactly which condition failed so
let's add this information to the error message. Because we now return an
allocated string from job_done_message_format(), make sure we strdup() the
other formats as well so the caller can safely free the string returned by
job_done_message_format().