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/_cloneDataView.js

@@ -0,0 +1,16 @@
+var cloneArrayBuffer = require('./_cloneArrayBuffer');
+
+/**
+ * Creates a clone of `dataView`.
+ *
+ * @private
+ * @param {Object} dataView The data view to clone.
+ * @param {boolean} [isDeep] Specify a deep clone.
+ * @returns {Object} Returns the cloned data view.
+ */
+function cloneDataView(dataView, isDeep) {
+  var buffer = isDeep ? cloneArrayBuffer(dataView.buffer) : dataView.buffer;
+  return new dataView.constructor(buffer, dataView.byteOffset, dataView.byteLength);
+}
+
+module.exports = cloneDataView;