Improve signing error message to developers

[zorgiepoo]

If we detect the wrong archive is being served (i.e, expected content length differs from archive length) we log this out to developers. If the app version in the archive (if available) also differs, we report this discrepancy as well. If the update archive looks the same but signing validation fails, we tell the developer the update may have not been signed correctly.

Fixes #2468

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested update succeed without errors
Tested update fail with matching content length and version (app bundle, pkg)
Tested update fail with non-matching content length and matching version (app bundle, pkg)
Tested update fail with non-matching content length and non-matching version (app bundle, pkg)

macOS version tested: 14.0 (23A344)