install: fail early if specifier expansion failed
Before:
systemd[1]: Assertion 'path' failed at src/shared/install.c:288, function install_changes_add(). Aborting.
systemd[1]: Caught <ABRT> from our own process.
systemd[1]: Caught <ABRT>, dumped core as pid 2525.
systemd[1]: Freezing execution
After:
Failed to enable unit: Invalid specifier in user-%J.service
Fixes #26467.
Follow-up for: f5a0162