gpt-auto-generator: emit warnings and fail if we fail to query mount points

gpt-auto-generator: emit warnings and fail if we fail to query mount points

Right now gpt-auto-generator will iterate through all mount entries, and
silently ignore failure to check if the mount point target is empty.
This can hide real errors (in particular from MAC), so instead let's warn
and return failure at the end if this happens. We will still iterate
over other candidates, so there should be no change in behaviour.

Logging is moved into path_is_busy() to avoid the duplication of the same
logging code in the two callers.