shared/install: provide a nicer error message for invalid WantedBy=/Required= values

$ build/systemctl --user cat badinstall
 # /home/zbyszek/.config/systemd/user/badinstall.service
[Service]
ExecStart=true

[Install]
WantedBy=asdf

$ build/systemctl --user enable badinstall
Failed to enable unit: "asdf" is not a valid unit name.

Fixes #4209.

(cherry picked from commit d7ceaf72)