fix(sandbox): propagate empty-message broker/fetch errors

Both bridges in the isolate used truthiness to detect host-side errors:

    if (response.error) throw new Error(response.error);   // broker
    if (result.error)   throw new Error(result.error);     // fetch

If a host handler ever threw `new Error('')`, err.message would be ''
(falsy), so { error: '' } was silently swallowed and the isolate saw
a successful null result. Existing call sites don't throw empty-message
errors, but the pattern was structurally unsafe.

Switch both to typeof check === 'string' and fall back to a default
message if the string is empty, so all host-reported errors propagate
into the isolate regardless of message content.

Made-with: Cursor

Author: icecrasher321

apps/sim/lib/execution/isolated-vm-worker.cjs

@@ -273,8 +273,8 @@ async function executeCode(request, executionId) {
           throw new Error('Invalid fetch response');
         }
 
-        if (result.error) {
-          throw new Error(result.error);
+        if (typeof result.error === 'string') {
+          throw new Error(result.error || 'Fetch failed');
         }
 
         // Create a Response-like object
@@ -775,7 +775,9 @@ async function executeTask(request, executionId) {
             );
             let response;
             try { response = JSON.parse(responseJson); } catch { throw new Error('Invalid broker response'); }
-            if (response.error) throw new Error(response.error);
+            if (typeof response.error === 'string') {
+              throw new Error(response.error || 'Broker call failed');
+            }
             return response.resultJson === undefined || response.resultJson === null
               ? null
               : JSON.parse(response.resultJson);