Errors don't surface to websockets for alloc exec/actions

[philrenaud]

In #19084 (comment), @pkazmierczak noticed that Permission Denied errors were not being returned to the opened websocket, resulting in a "Complete" state for the action, but no indication that a problem occurred.

Upon investigating, noticed that our exec functionality behaves similarly — we show a "The connection has closed." message int he exec terminal but no further indication that it had anything to do with permissions or otherwise.

The ws.WriteMessage call at

nomad/command/agent/alloc_endpoint.go

Lines 652 to 653 in a9d61ea

	websocket.FormatCloseMessage(toWsCode(codedErr.Code()), codedErr.Error()))
	}

indicates that we should have a way of returning with the error code, but it seems like the channel is closed before that has the chance to take place.