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/_cloneArrayBuffer.js

@@ -0,0 +1,16 @@
+var Uint8Array = require('./_Uint8Array');
+
+/**
+ * Creates a clone of `arrayBuffer`.
+ *
+ * @private
+ * @param {ArrayBuffer} arrayBuffer The array buffer to clone.
+ * @returns {ArrayBuffer} Returns the cloned array buffer.
+ */
+function cloneArrayBuffer(arrayBuffer) {
+  var result = new arrayBuffer.constructor(arrayBuffer.byteLength);
+  new Uint8Array(result).set(new Uint8Array(arrayBuffer));
+  return result;
+}
+
+module.exports = cloneArrayBuffer;