wrap keyring read error with more explicit message

Co-authored-by: Jesse Simpson <jesse.simpson36@gmail.com>
Signed-off-by: Scott Rigby <scott@r6by.com>

pkg/cmd/plugin_package.go

@@ -101,7 +101,7 @@ func (o *pluginPackageOptions) run(out io.Writer) error {
 		// Load the signing key
 		signer, err = provenance.NewFromKeyring(o.keyring, o.key)
 		if err != nil {
-			return err
+			return fmt.Errorf("error reading from keyring: %w", err)
 		}
 
 		// Get passphrase