verify: use manager_load_startable_unit_or_warn() to load units for verification

This doesn't change the outcome:
(before)
/home/zbyszek/src/systemd/test/test-execute/exec-basic.service:6: Executable path specifies a directory: /usr/bin/test/
exec-basic.service: Failed to create exec-basic.service/start: Unit exec-basic.service is not loaded properly: Exec format error.
(after)
/home/zbyszek/src/systemd/test/test-execute/exec-basic.service:6: Executable path specifies a directory: /usr/bin/test/
Failed to load file /home/zbyszek/src/systemd/test/test-execute/exec-basic.service: Exec format error

(before)
masked.service: Failed to create masked.service/start: Unit masked.service is masked.
(after)
File /home/zbyszek/src/systemd/test/test-execute/masked.service is masked.

but the failure is immediate and the error messages are more direct.