Indicate in web UI when forwarding a mouse event failed

[mtlynch]

Description

TinyPilot should give the user feedback about whether forwarding their mouse event failed. This is a little bit tricky, as mouse events can fire 50+ times per second, so we have to notify in a way that doesn't flood the screen.

What's the behavior that you expect?

TinyPilot gives a notification in the web UI that a mouse event failed.

What's happening instead?

There's no indication that the mouse event failed.

What are the steps to reproduce this behavior?

1. Connect TinyPilot to a target computer that's in console mode (no GUI)
2. Move the mouse cursor across the screen