Check for tier existence in in rpc plugin helpers

Текущее сообщение об ошибке (после влития МРа по тирам) совсем не годится. !1138 (comment 100597)

https://git.picodata.io/picodata/picodata/picodata/-/blob/master/src/vshard.rs?ref_type=heads#L28

https://git.picodata.io/picodata/picodata/picodata/-/blob/master/src/vshard.rs?ref_type=heads#L15

Фоллоуап для !1138 (merged) в рамках задачи #889 (closed)

changed milestone to %24.6 - Plugins and Tiers

added 1 deleted label

assigned to @kusancho

mentioned in merge request !1138 (merged)

mentioned in issue #889 (closed)

changed the description

mentioned in merge request !1289 (merged)

closed with merge request !1289 (merged)