If a SIGNING_KEY is specified in the config but not loaded, no error is printed to the log

[parnic]

Feature Description

We upgraded from 1.16.9 to 1.17.0 this weekend, and the changes from #20114 caused our Gitea-signed commits to start showing as unsigned; we didn't see any release notes that something had changed here. gitea.log contained this error only after trying to load a repo page that should have shown signed commits:

...mmit_verification.go:269:verifyWithGPGSettings() [E] Unable to get default signing key: failed to parse gpg key openpgp: invalid argument: no armored data found

but we did not understand that that meant "your config specifies a SIGNING_KEY, but we couldn't find it when we tried to load it up." If there is any way to add an explicit error on startup to this effect, it would greatly help others caught in the migration to 1.17.0 who didn't see the new notice at https://docs.gitea.io/en-us/signing/#signing_key. Something along these lines would be very helpful:

error: a gpg SIGNING_KEY was specified in config ($(key here)), but no key matching that id was found. Have you checked that a valid key exists? Keyring location: %(GNUPGHOME or [git].HOME_PATH location, whichever is being used)

Screenshots

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