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/flow.js

@@ -0,0 +1,27 @@
+var createFlow = require('./_createFlow');
+
+/**
+ * Creates a function that returns the result of invoking the given functions
+ * with the `this` binding of the created function, where each successive
+ * invocation is supplied the return value of the previous.
+ *
+ * @static
+ * @memberOf _
+ * @since 3.0.0
+ * @category Util
+ * @param {...(Function|Function[])} [funcs] The functions to invoke.
+ * @returns {Function} Returns the new composite function.
+ * @see _.flowRight
+ * @example
+ *
+ * function square(n) {
+ *   return n * n;
+ * }
+ *
+ * var addSquare = _.flow([_.add, square]);
+ * addSquare(1, 2);
+ * // => 9
+ */
+var flow = createFlow();
+
+module.exports = flow;