Improve some error messages with invalid indexes for REINDEX CONCURRENTLY

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