Re: Extract numeric filed in JSONB more effectively

An incompatible issue at error message level is found during test: 

create table jb(a jsonb);

insert into jb select '{"a": "a"}'::jsonb;

select (a->'a')::int4 from jb;

master:   ERROR:  cannot cast jsonb string to type integer

patch:  ERROR:  cannot cast jsonb string to type numeric

That's mainly because we first extract the field to numeric and

then cast it to int4 and the error raised at the first step and it

doesn't know the final type.  One way to fix it is adding a 2nd

argument for jsonb_finish_numeric for the real type, but

it looks weird and more suggestions on this would be good.