Export connection pool errors for public consumption

[LINKIWI]

The internal/pool package has some structured errors:

• ErrClosed
• ErrPoolExhausted
• ErrPoolTimeout

These errors may be propagated to the callsites of top-level commands on the connection. Exposing these errors in the public package layer makes it much easier for external code to detect and classify these errors, e.g. as:

err := ...
if errors.Is(err, redis.ErrPoolTimeout) {
    ...
}

This change proposes re-exporting the remaining ErrPool* errors, adding to ErrClosed which is already re-exported.

See also: #3381

[ndyakov]

LGTM, @LINKIWI thanks for the contribution