Improve handling of psql \watch's interval argument

Improve handling of psql \watch's interval argument

A failure in parsing the interval value defined in the \watch command
was silently switched to 1s of interval between two queries, which can
be confusing.  This commit improves the error handling, and a couple of
tests are added to check after:
- An incorrect value.
- An out-of-range value.
- A negative value.

A value of zero is able to work now, meaning that there is no interval
of time between two queries in a \watch loop.  No backpatch is done, as
it could break existing applications.

Author: Andrey Borodin
Reviewed-by: Kyotaro Horiguchi, Nathan Bossart, Michael Paquier
Discussion: https://postgr.es/m/CAAhFRxiZ2-n_L1ErMm9AZjgmUK=qS6VHb+0SaMn8sqqbhF7How@mail.gmail.com