replace jest with ava

node_modules/is-error/index.js

@@ -0,0 +1,23 @@
+'use strict';
+
+var objectToString = Object.prototype.toString;
+var getPrototypeOf = Object.getPrototypeOf;
+var ERROR_TYPE = '[object Error]';
+
+module.exports = function isError(err) {
+    if (typeof err !== 'object') {
+        return false;
+    }
+    if (err instanceof Error) {
+        // Accept `AssertionError`s from the `assert` module that ships
+        // with Node.js v6.1.0, compare issue #4.
+        return true;
+    }
+    while (err) {
+        if (objectToString.call(err) === ERROR_TYPE) {
+            return true;
+        }
+        err = getPrototypeOf(err);
+    }
+    return false;
+};