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/_baseInverter.js

@@ -0,0 +1,21 @@
+var baseForOwn = require('./_baseForOwn');
+
+/**
+ * The base implementation of `_.invert` and `_.invertBy` which inverts
+ * `object` with values transformed by `iteratee` and set by `setter`.
+ *
+ * @private
+ * @param {Object} object The object to iterate over.
+ * @param {Function} setter The function to set `accumulator` values.
+ * @param {Function} iteratee The iteratee to transform values.
+ * @param {Object} accumulator The initial inverted object.
+ * @returns {Function} Returns `accumulator`.
+ */
+function baseInverter(object, setter, iteratee, accumulator) {
+  baseForOwn(object, function(value, key, object) {
+    setter(accumulator, iteratee(value), key, object);
+  });
+  return accumulator;
+}
+
+module.exports = baseInverter;