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

src/main.ts

@@ -1,4 +1,5 @@
 import * as core from '@actions/core';
+import * as fs from "fs";
 import {uploadRelease} from "./edits";
 const {google} = require('googleapis');
 
@@ -27,6 +28,22 @@ async function run() {
             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);