Cancel repeating client build log on build error

refs https://github.com/TryGhost/Ghost/pull/9611
- #9611 added a repeating message to `grunt dev` output whilst waiting for a successful build but there was no handling for failed builds
- modify bgShell config to cancel the repeating message when there is output on the `stderr` pipe from `ember-cli`

Gruntfile.js

@@ -233,7 +233,10 @@ var config = require('./core/server/config'),
                             hasBuiltClient = true;
                         }
                     },
-                    stderr: true
+                    stderr: function (chunk) {
+                        hasBuiltClient = true;
+                        grunt.log.error(chunk);
+                    }
                 }
             },