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

node_modules/fs.realpath/package.json

@@ -0,0 +1,63 @@
+{
+  "_args": [
+    [
+      "fs.realpath@1.0.0",
+      "/Users/drew.heavner/Documents/Github/upload-google-play"
+    ]
+  ],
+  "_development": true,
+  "_from": "fs.realpath@1.0.0",
+  "_id": "fs.realpath@1.0.0",
+  "_inBundle": false,
+  "_integrity": "sha1-FQStJSMVjKpA20onh8sBQRmU6k8=",
+  "_location": "/fs.realpath",
+  "_phantomChildren": {},
+  "_requested": {
+    "type": "version",
+    "registry": true,
+    "raw": "fs.realpath@1.0.0",
+    "name": "fs.realpath",
+    "escapedName": "fs.realpath",
+    "rawSpec": "1.0.0",
+    "saveSpec": null,
+    "fetchSpec": "1.0.0"
+  },
+  "_requiredBy": [
+    "/glob"
+  ],
+  "_resolved": "https://registry.npmjs.org/fs.realpath/-/fs.realpath-1.0.0.tgz",
+  "_spec": "1.0.0",
+  "_where": "/Users/drew.heavner/Documents/Github/upload-google-play",
+  "author": {
+    "name": "Isaac Z. Schlueter",
+    "email": "i@izs.me",
+    "url": "http://blog.izs.me/"
+  },
+  "bugs": {
+    "url": "https://github.com/isaacs/fs.realpath/issues"
+  },
+  "dependencies": {},
+  "description": "Use node's fs.realpath, but fall back to the JS implementation if the native one fails",
+  "devDependencies": {},
+  "files": [
+    "old.js",
+    "index.js"
+  ],
+  "homepage": "https://github.com/isaacs/fs.realpath#readme",
+  "keywords": [
+    "realpath",
+    "fs",
+    "polyfill"
+  ],
+  "license": "ISC",
+  "main": "index.js",
+  "name": "fs.realpath",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/isaacs/fs.realpath.git"
+  },
+  "scripts": {
+    "test": "tap test/*.js --cov"
+  },
+  "version": "1.0.0"
+}