Make sure that GIN fast-insert and regular code paths enforce the same

tuple size limit.  Improve the error message for index-tuple-too-large
so that it includes the actual size, the limit, and the index name.
Sync with the btree occurrences of the same error.

Back-patch to 8.4 because it appears that the out-of-sync problem
is occurring in the field.

Teodor and Tom