Describe the bug

If you have a "bad generation" for certain reason, it can break your ability to fix by switching to a new generation.

To Reproduce
Steps to reproduce the behavior:

1. Accidentally set as the system profile, and then run a switch-to-configuration switch on a system build that is meant for an aarch64.
2. Fix this.
3. Try to activate a proper new generation.

Expected behavior
It works. And systemd-boot-builder.py skips any generations that are bad.

Additional context
The problem basically came down to initrd-append-secrets failing as bad of the 'bad generation' as systemd-boot-builder.py iterates over all system generations to build the boot entries.

I would suggest that maybe systemd-boot-builder.py should emit warnings but ultimately continue on failures such as failing to run initrd-append-secrets.