`timelib_astro_rise_set_altitude()` is not prepared to deal with non-
finite values (`nan`, `inf` and `-inf`) for `lon` and `lat`; instead
these trigger undefined behavior.  Thus we catch non-finite values
before even calling that timelib function; for `date_sun_info()` we
trigger `ValueError`s; for `date_sunrise()` and `date_sunset()` we
silently return `false`, since these functions will be sunsetted
anyway.