view: display small and invalid time ranges

view: display small and invalid time ranges

Instead of falling back to displaying the most recent messages
when there are too few (<200) or no messages in a time range,
show the small subset or return a 404 when there's no messages
at all.  This ought to be less confusing for users who want to
focus on a small subset of messages within a given timeframe.

Reported-by: Leah Neukirchen <leah@vuxu.org>
Link: https://public-inbox.org/meta/87zfbbpwhu.fsf@vuxu.org/