LibJS: Also throw exception when returning throw completion from await

...for now - the reason being that the AST breaks 'completion bubbling'
and returns a plain Value, and code at the call site relies on the VM
having an exception set when converting the plain value back into a
completion.

Fixes #11301.

Userland/Libraries/LibJS/Runtime/Completion.cpp

@@ -115,6 +115,10 @@ ThrowCompletionOr<Value> await(GlobalObject& global_object, Value value)
 
     if (success.value())
         return result;
+    // NOTE: This is temporary until we remove VM::exception(). It's required as callers of
+    //       AwaitExpression still need to check for an exception rather than a completion
+    //       type as long as ASTNode::execute() returns a plain Value.
+    vm.throw_exception(global_object, result);
     return throw_completion(result);
 }