ui: fix ui.configdate for invalid dates
a7dce526c462 introduced util._parsedate with the aim to be used in
ui.configdate but ui.configdate was using util.parsedate instead. It have the
impact of raising an AbortError in case of an invalid date instead of a
ConfigError exception. Fix ui.configdate to use the right function and add a
test for invalid dates.
Thanks to Yuya for the catch!
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors