core: slightly improve error message on load errors

core: slightly improve error message on load errors

Let's be a bit more helpful when refusing jobs on units that failed to
load properly. We already have explicit D-Bus errors for the error
conditions that are common and expected (such as "not found"), but for
the rest we so far generate a fairly cryptic message.

Let's try to be friendlier towards users and suggest what to do on such
errors.

Fixes: #16487