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/_cacheHas.js

@@ -0,0 +1,13 @@
+/**
+ * Checks if a `cache` value for `key` exists.
+ *
+ * @private
+ * @param {Object} cache The cache to query.
+ * @param {string} key The key of the entry to check.
+ * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+ */
+function cacheHas(cache, key) {
+  return cache.has(key);
+}
+
+module.exports = cacheHas;