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/ts-jest/dist/util/testing.js

@@ -0,0 +1,7 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+function mocked(item, _deep) {
+    if (_deep === void 0) { _deep = false; }
+    return item;
+}
+exports.mocked = mocked;