Also display a hopefully useful message with plymouth when GDM has failed to start Xorg 5 times (refs: #14521)

This ugly hack is needed because we have to keep count of the failures and kill
GDM ourselves, otherwise the gdm3 process is still running and the service is in
"active (running)" state, so OnFailure=tails-gdm-failed-to-start.service does
not take effect.