Reject nonzero day fields in AT TIME ZONE INTERVAL functions.

It's not sensible for an interval that's used as a time zone value to be
larger than a day.  When we changed the interval type to contain a separate
day field, check_timezone() was adjusted to reject nonzero day values, but
timetz_izone(), timestamp_izone(), and timestamptz_izone() evidently were
overlooked.

While at it, make the error messages for these three cases consistent.