allow stack traces in the browser

src/lib/errors.js

@@ -1,15 +1,23 @@
 var _ = require('./utils');
 var errors = module.exports;
 
+var canCapture = (typeof Error.captureStackTrace === 'function');
+var canStack = !!(new Error()).stack;
+
 function ErrorAbstract(msg, constructor) {
   this.message = msg;
 
   Error.call(this, this.message);
-  if (process.browser) {
-    this.stack = '';
-  } else {
+
+  if (canCapture) {
     Error.captureStackTrace(this, constructor);
   }
+  else if (canStack) {
+    this.stack = (new Error()).stack;
+  }
+  else {
+    this.stack = '';
+  }
 }
 errors._Abstract = ErrorAbstract;
 _.inherits(ErrorAbstract, Error);