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/_baseTimes.js

@@ -0,0 +1,20 @@
+/**
+ * The base implementation of `_.times` without support for iteratee shorthands
+ * or max array length checks.
+ *
+ * @private
+ * @param {number} n The number of times to invoke `iteratee`.
+ * @param {Function} iteratee The function invoked per iteration.
+ * @returns {Array} Returns the array of results.
+ */
+function baseTimes(n, iteratee) {
+  var index = -1,
+      result = Array(n);
+
+  while (++index < n) {
+    result[index] = iteratee(index);
+  }
+  return result;
+}
+
+module.exports = baseTimes;