Pool Close() should return error for compatibility with Client

[AliceInHunterland]

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

I'm always frustrated when I need to write additional wrappers. I want to have the pool's interface unified with the client's interface so no need to make wrappers like nspcc-dev/neo-go#3706 (comment).

Describe the solution you'd like

I'd like a returned error func (p *Pool) Close() error, similarly to func (c *Client) Close() error .

Describe alternatives you've considered

Additional context

Using SDK with both a pool and a single client.

[roman-khimov]

Unexpected. They're the same for real operations, but this one can also be a problem for pool users.