DateAPI date_select form inputs not marking incomplete dates as errors

Date_select fields that are not completely filled out are treated as empty and not flagged as being in error. For example, a non-required form field that's supposed to have a Y-m-d format but only has '2009', 'April' and '' selected will come back as FALSE rather than generating an error (like "date is required to complete date").

This is similar to #265076: Accept text input that omits hour & minute, even if the format expects it, in that incomplete elements are simply treated as empty - the difference is that I am expecting the input to be treated as an invalid rather than being silently ignored and treated as empty.