test-browser: improve unhandled error debugging (#8826)

* test-browser: improve unhandled error debugging

* Include message+stack+cause

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Co-authored-by: alxndrsn <alxndrsn>

Author: alxndrsn

bin/test-browser.js

@@ -259,6 +259,8 @@ async function startTest() {
       }
 
       console.log('Unhandled error in test page:', err);
+      console.log('  stack:', err.stack);
+      console.log('  cause:', err.cause);
       process.exit(1);
     });