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/_createOver.js

@@ -0,0 +1,27 @@
+var apply = require('./_apply'),
+    arrayMap = require('./_arrayMap'),
+    baseIteratee = require('./_baseIteratee'),
+    baseRest = require('./_baseRest'),
+    baseUnary = require('./_baseUnary'),
+    flatRest = require('./_flatRest');
+
+/**
+ * Creates a function like `_.over`.
+ *
+ * @private
+ * @param {Function} arrayFunc The function to iterate over iteratees.
+ * @returns {Function} Returns the new over function.
+ */
+function createOver(arrayFunc) {
+  return flatRest(function(iteratees) {
+    iteratees = arrayMap(iteratees, baseUnary(baseIteratee));
+    return baseRest(function(args) {
+      var thisArg = this;
+      return arrayFunc(iteratees, function(iteratee) {
+        return apply(iteratee, thisArg, args);
+      });
+    });
+  });
+}
+
+module.exports = createOver;