Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

lib/main.js

@@ -16,6 +16,7 @@ var __importStar = (this && this.__importStar) || function (mod) {
 };
 Object.defineProperty(exports, "__esModule", { value: true });
 const core = __importStar(require("@actions/core"));
+const fs = __importStar(require("fs"));
 const edits_1 = require("./edits");
 const { google } = require('googleapis');
 const auth = new google.auth.GoogleAuth({
@@ -42,6 +43,19 @@ function run() {
             else {
                 userFractionFloat = undefined;
             }
+            // Check release file
+            if (!fs.existsSync(releaseFile)) {
+                core.setFailed(`Unable to find release file @ ${releaseFile}`);
+                return;
+            }
+            if (whatsNewDir != undefined && !fs.existsSync(whatsNewDir)) {
+                core.setFailed(`Unable to find 'whatsnew' directory @ ${whatsNewDir}`);
+                return;
+            }
+            if (mappingFile != undefined && !fs.existsSync(mappingFile)) {
+                core.setFailed(`Unable to find 'mappingFile' @ ${mappingFile}`);
+                return;
+            }
             // Insure that the api can find our service account credentials
             core.exportVariable("GOOGLE_APPLICATION_CREDENTIALS", serviceAccountJson);
             const authClient = yield auth.getClient();