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/_setCacheHas.js

@@ -0,0 +1,14 @@
+/**
+ * Checks if `value` is in the array cache.
+ *
+ * @private
+ * @name has
+ * @memberOf SetCache
+ * @param {*} value The value to search for.
+ * @returns {number} Returns `true` if `value` is found, else `false`.
+ */
+function setCacheHas(value) {
+  return this.__data__.has(value);
+}
+
+module.exports = setCacheHas;