bootc fsck: drive typed errors down into the storage and boot code instead of anyhow::Error #248

From cgwalters' review of bootc-dev/bootc#2497 (bootc-dev/bootc#2497 (comment)): the readonly fsck report classifies problems into its own typed FindingCode enums (per subsystem: composefs, deployments, /boot, GRUB, EFI), but the code it inspects (composefs repository and deployment state, BLS/GRUB parsers, ESP handling) returns anyhow::Error, so fsck has to reimplement reads and parsing to get at structured errors. The real fix is for those modules to return typed errors that fsck (and status/upgrade) can match on, and for fsck to reuse them. This will be a sizable, incremental change; start once bootc-dev/bootc#2497 merges. Related: #173.

Generated-by: https://github.com/cgwalters/#llms