api: add a constant for "concurrent schema update"

[oleg-jukovec]

It would be nice to have a constant for the non-critical error: https://github.com/tarantool/go-tarantool/blob/f1d88dcd1108799cf26d952df921119fb06d5e2c/schema.go#L418

This will make it easier to repeat the call on the client side.

On the other hand, we can repeat the request ourselves in this case.