Issue 4579 - Wrap rejection reason in Error object

polyfill.js

Index: polyfill.js
===================================================================
--- a/polyfill.js
+++ b/polyfill.js
@@ -55,16 +55,18 @@
 
       if (!object)
         return;
     }
 
     let func = object[name];
     object[name] = function(...args)
     {
+      let callStack = new Error().stack;
+
       if (typeof args[args.length - 1] == "function")
         return func.apply(object, args);
 
       // If the last argument is undefined, we drop it from the list assuming
       // it stands for the optional callback. We must do this, because we have
       // to replace it with our own callback. If we simply append our own
       // callback to the list, it won't match the signature of the function and
       // will cause an exception.
@@ -72,19 +74,33 @@
         args.pop();
 
       return new Promise((resolve, reject) =>
       {
         func.call(object, ...args, result =>
         {
           let error = browser.runtime.lastError;
           if (error)
+          {
+            // runtime.lastError is already an Error instance on Edge, while on
+            // Chrome it is a plain object with only a message property.
+            if (!(error instanceof Error))
+            {
+              error = new Error(error.message);
+
+              // Add a more helpful stack trace.
+              error.stack = callStack;
+            }
+
             reject(error);
+          }
           else
+          {
             resolve(result);
+          }
         });
       });
     };
   }
 
   function shouldWrapAPIs()
   {
     try