Use retry notifications for crosspost errors

[ekzyis]

Is your feature request related to a problem? Please describe.

Someone just reported that they were confused by a crosspost error. They initially were worried that their SN post failed.

Since we already have retry notifications for failed zaps, we could do the same for crosspost errors.

Describe the solution you'd like

Have a notification if a crosspost to a relay failed.

Describe alternatives you've considered

Doing only this: #1285

Additional context

Since crosspost errors are client-side only, this could use the old code for client notifications in 94cce91.