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/_createInverter.js

@@ -0,0 +1,17 @@
+var baseInverter = require('./_baseInverter');
+
+/**
+ * Creates a function like `_.invertBy`.
+ *
+ * @private
+ * @param {Function} setter The function to set accumulator values.
+ * @param {Function} toIteratee The function to resolve iteratees.
+ * @returns {Function} Returns the new inverter function.
+ */
+function createInverter(setter, toIteratee) {
+  return function(object, iteratee) {
+    return baseInverter(object, setter, toIteratee(iteratee), {});
+  };
+}
+
+module.exports = createInverter;