goat account service-auth-local: nicer errors

cmd/goat/account.go

···
 	}
 	privkey, err := crypto.ParsePrivateMultibase(privStr)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed parsing private key: %w", err)
 	}
 
 	issString := cctx.String("iss")
···
 
 	token, err := auth.SignServiceAuth(iss, aud, duration, lxm, privkey)
 	if err != nil {
-		return err
+		return fmt.Errorf("failed signing token: %w", err)
 	}
 
 	fmt.Println(token)