The build of initrd-secrets can routinely fail for old boot entries
if the secrets have been removed or renamed in a later generation.
This always happens for generation 1, because it's built from the
NixOS installer and the paths differs by the mount point (i.e. /mnt).
The error is very confusing because it fails to mention it's about
an older generation and that it's somewhat harmless.
This commit turns the error into a warning for all generations but the
current, adds the name of the failed entry to the message and a note
explaining why it can happen.
eccc1e5bf4