Properly report invalid quoted strings

Properly report invalid quoted strings

$ systemd-analyze verify trailing-g.service
[./trailing-g.service:2] Trailing garbage, ignoring.
trailing-g.service lacks ExecStart setting. Refusing.
Error: org.freedesktop.systemd1.LoadFailed: Unit trailing-g.service failed to load: Invalid argument.
Failed to create trailing-g.service/start: Invalid argument