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/_setCacheAdd.js

@@ -0,0 +1,19 @@
+/** Used to stand-in for `undefined` hash values. */
+var HASH_UNDEFINED = '__lodash_hash_undefined__';
+
+/**
+ * Adds `value` to the array cache.
+ *
+ * @private
+ * @name add
+ * @memberOf SetCache
+ * @alias push
+ * @param {*} value The value to cache.
+ * @returns {Object} Returns the cache instance.
+ */
+function setCacheAdd(value) {
+  this.__data__.set(value, HASH_UNDEFINED);
+  return this;
+}
+
+module.exports = setCacheAdd;