Specifically, move those functions that depend on ereport()
from jsonapi.c to jsonfuncs.c, in preparation for allowing
jsonapi.c to be used from frontend code.

A few cases where elog(ERROR, ...) is used for can't-happen
conditions are left alone; we can handle those in some other
way in frontend code.

Reviewed by Mark Dilger and Andrew Dunstan.

Discussion: http://postgr.es/m/CA+TgmoYfOXhd27MUDGioVh6QtpD0C1K-f6ObSA10AWiHBAL5bA@mail.gmail.com