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/_baseIsArrayBuffer.js

@@ -0,0 +1,17 @@
+var baseGetTag = require('./_baseGetTag'),
+    isObjectLike = require('./isObjectLike');
+
+var arrayBufferTag = '[object ArrayBuffer]';
+
+/**
+ * The base implementation of `_.isArrayBuffer` without Node.js optimizations.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is an array buffer, else `false`.
+ */
+function baseIsArrayBuffer(value) {
+  return isObjectLike(value) && baseGetTag(value) == arrayBufferTag;
+}
+
+module.exports = baseIsArrayBuffer;