api: add a constant for "concurrent schema update"

[oleg-jukovec]

It would be nice to have a constant for the non-critical error:

go-tarantool/schema.go

Line 418 in f1d88dc

return Schema{}, errors.New("concurrent schema update")

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.