swarm: Add `ConnectionError` to `FromSwarm::ConnectionClosed`

[dariusc93]

Description

Expose ConnectionError to FromSwarm::ConnectionClosed to allow behaviours to see the error that is related to the closure of the connection.

Motivation

When a connection close due to an error, the behaviour would not know or be able to act on that error, while swarm would be notified of the error.

Current Implementation

FromSwarm::ConnectionClosed does not report or provide ConnectionError.

Are you planning to do it yourself in a pull request ?

Yes

[dariusc93]

See #5039