[#90867] Mention MM/DD/YYYY format when validation fails

A quick followup to https://github.com/tablexi/nucore-open/pull/222 that gives users some feedback when manually entering a journal date in something other than MM/DD/YYYY format. This should not come into play when choosing a date with the datepicker widget.
I wanted to hang on to using a regex match, because the actual error message is longer, and this test in particular cares about "may not be blank" but not necessarily the part that mentions formatting.