Fix a bad condition

#2123 (comment)

Author: gaearon

packages/react-error-overlay/src/components/overlay.js

@@ -52,7 +52,7 @@ function createOverlay(
   applyStyles(header, headerStyle);
 
   // Make message prettier
-  let finalMessage = message.match(/^\w*:/) ? name + ': ' + message : message;
+  let finalMessage = message.match(/^\w*:/) ? message : name + ': ' + message;
   finalMessage = finalMessage
     // TODO: maybe remove this prefix from fbjs?
     // It's just scaring people