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

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.)

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/653aa603f501aa6e4865105a928cd13082ee7152

Modified Files
--------------
src/backend/executor/execExpr.c      | 8 ++++++++
src/backend/optimizer/util/clauses.c | 5 +++--
src/backend/parser/parse_node.c      | 6 ++++++
src/backend/utils/cache/lsyscache.c  | 7 ++-----
src/test/regress/expected/arrays.out | 2 ++
5 files changed, 21 insertions(+), 7 deletions(-)