Reject invalid string in --date argument

Reject invalid string in --date argument

As parse_date_or_fatal() currently uses atof(), which just returns 0
if the string isn't a valid number, it can never actually be fatal
and will just use "1970-01-01 00:00:00" as the time if the string is
specified wrong.

Fix that by using float() directly so ValueError() is raised.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: adjust commit summary]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>