add error message for authenticator not found

src/main.zig

@@ -23,6 +23,9 @@ pub fn main() !void {
                 // do nothing, error message is already written (because the message contains the name of the unknown parameter)
             },
             ArgumentError.InvalidOtpAuthUrl => {},
+            ArgumentError.AuthenticatorNotFound => {
+                try std.io.getStdErr().writer().print("authenticator not found\n", .{});
+            },
             else => |leftover_err| {
                 try std.io.getStdErr().writer().print("{?}\n", .{leftover_err});
             },