Allow Promise to display error strings and not just error objects.

Summary: Closes https://github.com/facebook/react-native/pull/9989

Reviewed By: bestander

Differential Revision: D4147256

Pulled By: hramos

fbshipit-source-id: 62d49b592391bad434062e3c0d9c8287842664a8

Libraries/Promise.js

@@ -13,14 +13,26 @@
 
 const Promise = require('fbjs/lib/Promise.native');
 
+const prettyFormat = require('pretty-format');
+
 if (__DEV__) {
   require('promise/setimmediate/rejection-tracking').enable({
     allRejections: true,
     onUnhandled: (id, error = {}) => {
-      const {message = null, stack = null} = error;
+      let message: string;
+      let stack: ?string;
+
+      const stringValue = Object.prototype.toString.call(error);
+      if (stringValue === '[object Error]') {
+        message = Error.prototype.toString.call(error);
+        stack = error.stack;
+      } else {
+        message = prettyFormat(error);
+      }
+
       const warning =
         `Possible Unhandled Promise Rejection (id: ${id}):\n` +
-        (message == null ? '' : `${message}\n`) +
+        `${message}\n` +
         (stack == null ? '' : stack);
       console.warn(warning);
     },

package.json

@@ -184,6 +184,7 @@
     "opn": "^3.0.2",
     "optimist": "^0.6.1",
     "plist": "^1.2.0",
+    "pretty-format": "^4.2.1",
     "promise": "^7.1.1",
     "react-clone-referenced-element": "^1.0.1",
     "react-devtools-core": "^2.0.8",