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.)

src/backend/executor/execExpr.c

@@ -2536,6 +2536,14 @@ ExecInitSubscriptingRef(ExprEvalStep *scratch, SubscriptingRef *sbsref,
 
 	/* Look up the subscripting support methods */
 	sbsroutines = getSubscriptingRoutines(sbsref->refcontainertype, NULL);
+	if (!sbsroutines)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("cannot subscript type %s because it does not support subscripting",
+						format_type_be(sbsref->refcontainertype)),
+				 state->parent ?
+				 executor_errposition(state->parent->state,
+									  exprLocation((Node *) sbsref)) : 0));
 
 	/* Allocate sbsrefstate, with enough space for per-subscript arrays too */
 	sbsrefstate = palloc0(MAXALIGN(sizeof(SubscriptingRefState)) +