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/_baseToPairs.js

@@ -0,0 +1,18 @@
+var arrayMap = require('./_arrayMap');
+
+/**
+ * The base implementation of `_.toPairs` and `_.toPairsIn` which creates an array
+ * of key-value pairs for `object` corresponding to the property names of `props`.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @param {Array} props The property names to get values for.
+ * @returns {Object} Returns the key-value pairs.
+ */
+function baseToPairs(object, props) {
+  return arrayMap(props, function(key) {
+    return [key, object[key]];
+  });
+}
+
+module.exports = baseToPairs;