Try the following:
The final %-G%.%# acts as a "catch all" clause, that ignores all lines, that are not matched by previous expressions.
The following should also work:
:set errorformat=%A%f:%l:%c:%m,%-G\\s%#,%-G%*\\d\ problem%.%#
%-G\\s%# ignores all lines that contains zero or more whitespaces. This ...