Update some timestamp[tz] functions to use soft-error reporting

Update some timestamp[tz] functions to use soft-error reporting

This commit updates two functions that convert "timestamptz" to
"timestamp", and vice-versa, to use the soft error reporting rather than
a their own logic to do the same.  These are now named as follows:
- timestamp2timestamptz_safe()
- timestamptz2timestamp_safe()

These functions were suffixed with "_opt_overflow", previously.

This shaves some code, as it is possible to detect how a timestamp[tz]
overflowed based on the returned value rather than a custom state.  It
is optionally possible for the callers of these functions to rely on the
error generated internally by these functions, depending on the error
context.

Similar work has been done in d03668ea0566 and 4246a977bad6.

Reviewed-by: Amul Sul <sulamul@gmail.com>
Discussion: https://postgr.es/m/aS09YF2GmVXjAxbJ@paquier.xyz