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/parser/parse_node.c

@@ -271,6 +271,12 @@ transformContainerSubscripts(ParseState *pstate,
 	 * functions and typelem.
 	 */
 	sbsroutines = getSubscriptingRoutines(containerType, &elementType);
+	if (!sbsroutines)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATATYPE_MISMATCH),
+				 errmsg("cannot subscript type %s because it does not support subscripting",
+						format_type_be(containerType)),
+				 parser_errposition(pstate, exprLocation(containerBase))));
 
 	/*
 	 * Detect whether any of the indirection items are slice specifiers.