Remove the `ErrorSlot` type

[teor2345]

Motivation

We're seeing "failed servers must set their error slot" panics when Zebra is overloaded.

This panic is caused by the design of the Client connection code. Currently, the Client tasks share an ErrorSlot mutex, which must be updated before they exit.

This ticket removes the ErrorSlot type, after we stop using it:

• #4733
• #4734

Designs

Remove the ErrorSlot type, and any methods that use it.

Here are some cleanups we might be able to do:

• delete Connection's State::Failed and fail_with
• delete the ErrorSlot

Some of this cleanup might happen in earlier tickets.

Related Work

This is part of:

• #3263

[teor2345]

We're making network issues a lower priority for now.