Returned when a Tempo expression cannot be stored in a PostgreSQL
range type, its :reason saying why.
See the storage contract guide for the full list of cases. The
reasons are :qualification (an uncertain or approximate value),
:non_gregorian_calendar, :multi_valued_slot (several spans where
a range holds one), :partial_resolution (a value with no year),
:fully_unbounded (../..), :open_ended_member (a recurrence
with no end, where a multirange needs a last span),
:unsupported_endpoint (a range bound, or a stretch, with no place
on the time line) and :frame (a floating value for a column on the
UTC line, or a value in a zone or at a time shift for a floating
column).