Provide an error cursor for "can't subscript" error messages.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 653aa603f501aa6e4865105a928cd13082ee7152
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2020-12-11T23:58:21Z
Releases: 14.0
Provide an error cursor for "can't subscript" error messages.

Commit c7aba7c14 didn't add this, but after more fooling with the
feature I feel that it'd be useful.  To make this possible, refactor
getSubscriptingRoutines() so that the caller is responsible for
throwing any error.  (In clauses.c, I just chose to make the
most conservative assumption rather than throwing an error.  We don't
expect failures there anyway really, so the code space for an error
message would be a poor investment.)

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