add test for initial error matcher

lib/error_matcher.js

@@ -1,6 +1,18 @@
 "use strict";
 Object.defineProperty(exports, "__esModule", { value: true });
-exports.error_matchers = [
-    [32, new RegExp("No JavaScript or TypeScript code found\\."), 'foo bar']
-];
+// exported only for testing purposes
+exports.namedMatchersForTesting = {
+    /*
+    In due course it may be possible to remove the regex, if/when javascript also exits with code 32.
+     For context see https://github.com/github/semmle-code/pull/36921
+    */
+    noSourceCodeFound: [
+        32,
+        new RegExp("No JavaScript or TypeScript code found\\."),
+        `No source code was seen during the build. Please see...
+https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/troubleshooting-code-scanning#no-code-found-during-the-build`
+    ],
+};
+// we collapse the matches into an array for use in exec_wrapper
+exports.errorMatchers = Object.values(exports.namedMatchersForTesting);
 //# sourceMappingURL=error_matcher.js.map