Restructure soft-error handling in formatting.c.

Restructure soft-error handling in formatting.c.

Replace the error trapping scheme introduced in 5bc450629 with our
shiny new errsave/ereturn mechanism.  This doesn't have any real
functional impact (although I think that the new coding is able
to report a few more errors softly than v15 did).  And I doubt
there's any measurable performance difference either.  But this
gets rid of an ad-hoc, one-of-a-kind design in favor of a mechanism
that will be widely used going forward, so it should be a net win
for code readability.

Discussion: https://postgr.es/m/3bbbb0df-7382-bf87-9737-340ba096e034@postgrespro.ru