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.

src/core/dbus-manager.c

@@ -1902,6 +1902,12 @@ static int install_error(
                                               "Unit %s is transient or generated.", changes[i].path);
                         goto found;
 
+                case -EUCLEAN:
+                        r = sd_bus_error_setf(error, BUS_ERROR_BAD_UNIT_SETTING,
+                                              "\"%s\" is not a valid unit name.",
+                                              changes[i].path);
+                        goto found;
+
                 case -ELOOP:
                         r = sd_bus_error_setf(error, BUS_ERROR_UNIT_LINKED,
                                               "Refusing to operate on alias name or linked unit file: %s",