Give each failure one error value (closes #113)

internal/cli drops its copies of vault.ErrSecretNotFound, ErrVaultNotFound,
ErrVersionNotFound and ErrSecretExists and of the secret package's keychain
and Secure Enclave errors, and its second error for an unknown unlocker
type, an invalid mnemonic, a length below 1, an unsupported secret type and
an oversized secret. vault.ErrNilValueBuffer becomes
secret.ErrNilValueBuffer. Every error of secret.ReadPassphrase wraps
ErrPassphraseNotRead. ResolveGPGKeyFingerprint returns ErrGPGKeyNotFound for
a key the keyring lacks. storeInKeychain returns errNilDataBuffer. bip85's
ErrPasswordTooShort and ErrEncodedTooShort go with their unreachable checks,
as does the macOS check in macOS-only code. Tests that matched these
errors' text use errors.Is.

Model: opus-5-5

internal/secret/pgpunlocker.go

@@ -18,6 +18,10 @@ import (
 	"github.com/spf13/afero"
 )
 
+// gpgNoPublicKeyStatus is the status line gpg writes when it has no key for
+// the ID it was asked to list: 9 is gpg's error code for "No public key".
+const gpgNoPublicKeyStatus = "[GNUPG:] ERROR keylist.getkey 9\n"
+
 var (
 	errGPGKeyIDEmpty    = errors.New("GPG key ID cannot be empty")
 	errInvalidGPGKeyID  = errors.New("invalid GPG key ID format")
@@ -25,6 +29,10 @@ var (
 	errNilDataBuffer    = errors.New("data buffer is nil")
 )
 
+// ErrGPGKeyNotFound is returned by ResolveGPGKeyFingerprint for a key ID
+// that matches no key in the GPG keyring.
+var ErrGPGKeyNotFound = errors.New("GPG key not found")
+
 // Variables to allow overriding in tests
 var (
 	// GPGEncryptFunc is the function used for GPG encryption
@@ -367,14 +375,20 @@ func ResolveGPGKeyFingerprint(keyID string) (string, error) {
 		return "", fmt.Errorf("invalid GPG key ID: %w", err)
 	}
 
-	// Use GPG to get the full fingerprint for the key
+	// Use GPG to get the full fingerprint for the key. --status-fd 1 adds
+	// gpg's status lines to the output.
 	cmd := exec.CommandContext( //nolint:gosec // G204: keyID validated above
 		context.Background(),
-		"gpg", "--list-keys", "--with-colons", "--fingerprint", keyID,
+		"gpg", "--status-fd", "1",
+		"--list-keys", "--with-colons", "--fingerprint", keyID,
 	)
 
 	output, err := cmd.Output()
 	if err != nil {
+		if strings.Contains(string(output), gpgNoPublicKeyStatus) {
+			return "", fmt.Errorf("%w: %s", ErrGPGKeyNotFound, keyID)
+		}
+
 		return "", fmt.Errorf("failed to resolve GPG key fingerprint: %w", err)
 	}