Use dialog to report invalid app.

atom/browser/default_app/main.js

@@ -1,4 +1,5 @@
 var app = require('app');
+var dialog = require('dialog');
 var path = require('path');
 var optimist = require('optimist');
 
@@ -17,8 +18,14 @@ if (argv._.length > 0) {
     require(path.resolve(argv._[0]));
   } catch(e) {
     if (e.code == 'MODULE_NOT_FOUND') {
-      console.error(e.stack);
-      console.error('Specified app is invalid');
+      app.focus();
+      dialog.showMessageBox({
+        type: 'warning',
+        buttons: ['OK'],
+        title: 'Error opening app',
+        message: 'The app provided is not a valid atom-shell app, please read the docs on how to write one:',
+        detail: 'https://github.com/atom/atom-shell/tree/master/docs'
+      });
       process.exit(1);
     } else {
       throw e;