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/_createAggregator.js

@@ -0,0 +1,23 @@
+var arrayAggregator = require('./_arrayAggregator'),
+    baseAggregator = require('./_baseAggregator'),
+    baseIteratee = require('./_baseIteratee'),
+    isArray = require('./isArray');
+
+/**
+ * Creates a function like `_.groupBy`.
+ *
+ * @private
+ * @param {Function} setter The function to set accumulator values.
+ * @param {Function} [initializer] The accumulator object initializer.
+ * @returns {Function} Returns the new aggregator function.
+ */
+function createAggregator(setter, initializer) {
+  return function(collection, iteratee) {
+    var func = isArray(collection) ? arrayAggregator : baseAggregator,
+        accumulator = initializer ? initializer() : {};
+
+    return func(collection, setter, baseIteratee(iteratee, 2), accumulator);
+  };
+}
+
+module.exports = createAggregator;