make ErrorMatcher an object rather than a tuple

src/error-matcher.ts

@@ -1,17 +1,23 @@
 
-export type ErrorMatcher = [number|null, RegExp|null, string];
+// defines properties to match against the result of executed commands,
+// and a custom error to return when a match is found
+export interface ErrorMatcher {
+  exitCode?: number;     // exit code of the run process
+  outputRegex?: RegExp;  // pattern to match against either stdout or stderr
+  message: string;       // the error message that will be thrown for a matching process
+}
 
 // exported only for testing purposes
 export const namedMatchersForTesting: { [key: string]: ErrorMatcher } = {
   /*
   In due course it may be possible to remove the regex, if/when javascript also exits with code 32.
   */
-  noSourceCodeFound: [
-    32,
-    new RegExp("No JavaScript or TypeScript code found\\."),
-    "No code found during the build. Please see:\n" +
-    "https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/troubleshooting-code-scanning#no-code-found-during-the-build"
-  ],
+  noSourceCodeFound: {
+    exitCode: 32,
+    outputRegex: new RegExp("No JavaScript or TypeScript code found\\."),
+    message: "No code found during the build. Please see:\n" +
+             "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 execErrorCatcher