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/lodash/_safeGet.js

@@ -0,0 +1,21 @@
+/**
+ * Gets the value at `key`, unless `key` is "__proto__" or "constructor".
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @param {string} key The key of the property to get.
+ * @returns {*} Returns the property value.
+ */
+function safeGet(object, key) {
+  if (key === 'constructor' && typeof object[key] === 'function') {
+    return;
+  }
+
+  if (key == '__proto__') {
+    return;
+  }
+
+  return object[key];
+}
+
+module.exports = safeGet;