Improve some error messages with invalid indexes for REINDEX CONCURRENTLY
An invalid index is skipped when doing REINDEX CONCURRENTLY at table
level, with INDEX_CORRUPTED used as errcode. This is confusing,
because an invalid index could exist after an interruption. The errcode
is switched to OBJECT_NOT_IN_PREREQUISITE_STATE instead, as per a
suggestion from Andres Freund.
While on it, the error messages are reworded, and a hint is added,
telling how to rebuild an invalid index in this case. This has been
suggested by Noah Misch.
Discussion: https://postgr.es/m/
20231118230958.4fm3fhk4ypshxopa@awork3.anarazel.de