Convert datetime input functions to use "soft" error reporting.

Convert datetime input functions to use "soft" error reporting.

This patch converts the input functions for date, time, timetz,
timestamp, timestamptz, and interval to the new soft-error style.
There's some related stuff in formatting.c that remains to be
cleaned up, but that seems like a separable project.

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