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/_createBaseFor.js

@@ -0,0 +1,25 @@
+/**
+ * Creates a base function for methods like `_.forIn` and `_.forOwn`.
+ *
+ * @private
+ * @param {boolean} [fromRight] Specify iterating from right to left.
+ * @returns {Function} Returns the new base function.
+ */
+function createBaseFor(fromRight) {
+  return function(object, iteratee, keysFunc) {
+    var index = -1,
+        iterable = Object(object),
+        props = keysFunc(object),
+        length = props.length;
+
+    while (length--) {
+      var key = props[fromRight ? length : ++index];
+      if (iteratee(iterable[key], key, iterable) === false) {
+        break;
+      }
+    }
+    return object;
+  };
+}
+
+module.exports = createBaseFor;