Improve decryption failure messages with more granular failures

[colelawrence]

I had an AI auto-complete accidentally update a single line in my fnox.toml file, and I noticed surprisingly, that I got the errors

$ fnox edit --verbose
2025-11-28T16:17:52.420334Z DEBUG fnox: Using config file: fnox.toml
2025-11-28T16:17:52.420751Z DEBUG fnox::commands::edit: Starting enhanced edit with profile: default
2025-11-28T16:17:52.420850Z DEBUG fnox::commands::edit: Decrypting 6 secrets
Error: fnox::encryption::age::decrypt_failed

  × Age decryption failed
  help: Ensure you have the correct age identity file or FNOX_AGE_KEY environment variable set

From the looks of this, it looks like my key is wrong even though it was actually only one of the secrets that failed to decrypt because that was the one that was accidentally updated.

I would have liked to have seen a more explicit error that could say, "These secrets were decrypted correctly," and "These secrets were not."