Re: Error-safe user functions

I wrote:
> Seems like everybody's okay with errsave. I'll make a v2 in a
> little bit. I'd like to try updating array_in and/or record_in
> just to verify that indirection cases work okay, before we consider
> the design to be set.

v2 as promised, incorporating the discussed renamings as well as some
follow-on ones (ErrorReturnContext -> ErrorSaveContext, notably).

I also tried moving the struct into a new header file, miscnodes.h
after Andrew's suggestion upthread. That seems at least marginally
cleaner than putting it in nodes.h, although I'm not wedded to this
choice.

I was really glad that I took the trouble to update some less-trivial
input functions, because I learned two things:

* It's better if InputFunctionCallSafe will tolerate the case of not
being passed an ErrorSaveContext. In the COPY hack it felt worthwhile
to have completely separate code paths calling InputFunctionCallSafe
or InputFunctionCall, but that's less appetizing elsewhere.

* There's a crying need for a macro that wraps up errsave() with an
immediate return. Hence, ereturn() is reborn from the ashes. I hope
Robert won't object to that name if it *does* do a return.

I feel pretty good about this version; it seems committable if there
are not objections. Not sure if we should commit 0003 like this,
though.

regards, tom lane