ExtensionImages: log explicit error when extension-release metadata does not match

When an ExtensionImages= extension-release metadata does not match, the
log messages (unless debug level is set) are pretty much incomprehensible:

systemd[463]: run-u11.service: Failed to set up mount namespacing: /run/systemd/unit-extensions/0: Stale file handle
systemd[463]: run-u11.service: Failed at step NAMESPACE spawning /usr/bin/echo: Stale file handle

Add an explicit log message if we get ESTALE from the dissect code, to
make it clear what's happening without needing to enable debugging:

systemd[463]: Failed to mount image /tmp/app3.raw, extension-release metadata does not match the lower layer's: ID=debian VERSION_ID=11 SYSEXT_LEVEL=11