udev-rules: downgrade error about non-system user/group in OWNER=/GROUP=

udev-rules: downgrade error about non-system user/group in OWNER=/GROUP=

This reverts (in sprit) commit f5cdf9515aceca2e91f9a33b74267e0cf5a5b7e8,
"udev-rules: ignore non-system user/group in OWNER=/GROUP=".

The original change was done to clean up a situation where we added a
new group, but the group could already have been used for some other
purposes, and now the some unexpected entity would own the device.
Unfortunately, this check doesn't really address the issue, since the
existing account might as well be a system account, which might be
equally bad. In addition, this change is a big compatiblity break,
causing existing rules to stop working. Since quite a lot of systems
have local configuration to assign devices to users for various
purposes, this is very noticable to users. In a way, the original change
to add a new group was the compat break, and follow-up patch to cahnge
the rule parsing evolved a small compat break into a much bigger one.
There is merit to the change though, since device nodes shouldn't be
owned by users and groups and different mechanisms should be used
instead. To avoid breaking users systems, and since the original goal
cannot be achieved by this patch, let's downgrade this to a warning
to guide users towards different solutions.