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/@babel/template/lib/string.js

@@ -0,0 +1,24 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.default = stringTemplate;
+
+var _options = require("./options");
+
+var _parse = _interopRequireDefault(require("./parse"));
+
+var _populate = _interopRequireDefault(require("./populate"));
+
+function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }
+
+function stringTemplate(formatter, code, opts) {
+  code = formatter.code(code);
+  let metadata;
+  return arg => {
+    const replacements = (0, _options.normalizeReplacements)(arg);
+    if (!metadata) metadata = (0, _parse.default)(formatter, code, opts);
+    return formatter.unwrap((0, _populate.default)(metadata, replacements));
+  };
+}