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/_getWrapDetails.js

@@ -0,0 +1,17 @@
+/** Used to match wrap detail comments. */
+var reWrapDetails = /\{\n\/\* \[wrapped with (.+)\] \*/,
+    reSplitDetails = /,? & /;
+
+/**
+ * Extracts wrapper details from the `source` body comment.
+ *
+ * @private
+ * @param {string} source The source to inspect.
+ * @returns {Array} Returns the wrapper details.
+ */
+function getWrapDetails(source) {
+  var match = source.match(reWrapDetails);
+  return match ? match[1].split(reSplitDetails) : [];
+}
+
+module.exports = getWrapDetails;