generate_appcast does not return an error if private key doesn't exist

[mborsten]

Summary

When running generate_appcas and the private key doesn't exist in the keychain, it does print out a warning, but it still generates an update xml:

Warning: Private key for account ed25519 not found in the Keychain (-25300). Please run the generate_keys tool
Warning: could not sign **** due to lack of private EdDSA key
Wrote 1 new update, updated 0 existing updates, and removed 0 old updates in appcast.xml

The return value of the proces is also 0

Possible Fix

The proces should by default abort and have a non-0 return value. This is especially important when in CI/CD, where you want the deploy to abort if the singing didn't work. There could be an option to skip signing, or make signing optional.

[zorgiepoo]

If the update has a public EdDSA key, and generate_appcast can't sign the update, this should indeed be a hard error.