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/ajv/lib/cache.js

@@ -0,0 +1,26 @@
+'use strict';
+
+
+var Cache = module.exports = function Cache() {
+  this._cache = {};
+};
+
+
+Cache.prototype.put = function Cache_put(key, value) {
+  this._cache[key] = value;
+};
+
+
+Cache.prototype.get = function Cache_get(key) {
+  return this._cache[key];
+};
+
+
+Cache.prototype.del = function Cache_del(key) {
+  delete this._cache[key];
+};
+
+
+Cache.prototype.clear = function Cache_clear() {
+  this._cache = {};
+};