When listing all packages, the purpose is to get a quick look at what's
installed and not to scrutinize the validity of each .pc file. To see
errors from the parser during --list-all, the user can just add
--print-errors.
The intention was that errors would be printed for all output options
besides --exists and --atleast/exact/max-version, which are intended to
operate silently. Since want_exists is always set for these latter
options, we can simply use that as the condition and catch all other
output options automatically.
Freedesktop #54390 (https://bugs.freedesktop.org/show_bug.cgi?id=54390)
