Relax macOS notarisation check to print a warning

This makes it a bit more friendly to build macOS Electron builds without a
notarisation config set up by only warning when it is missing.

scripts/electron_afterSign.js

@@ -10,7 +10,10 @@ exports.default = async function(context) {
         // from the keychain, so we need to get it from the environment.
         const userId = process.env.NOTARIZE_APPLE_ID;
         if (userId === undefined) {
-            throw new Error("User ID not found. Set NOTARIZE_APPLE_ID.");
+            console.warn(
+                "Skipping notarisation: User ID not found, set NOTARIZE_APPLE_ID.",
+            );
+            return;
         }
 
         console.log("Notarising macOS app. This may be some time.");