Disallow DateTime parsing of strings without digits

plus more test cases

Something like 'x' will be parsed as current time, with
precision of seconds which we don't support. Users then see
error "Precision higher than 'DAY' is not supported'.

Bug: 64662
Change-Id: I1650db7bd28b18835e5fe6f6dba520fff68c3c82
Merge "Disallow DateTime parsing of strings without digits"