busctl: verify args early and always print results to stdout

busctl: verify args early and always print results to stdout

We would print the error sometimes to stdout and sometimes to stderr. It *is*
useful to get the message if one of the names is not found on the bus to
stdout, so that this shows out in the pager. So let's do verification of args
early to catch invalid arguments, and then if we receive an error over the bus
(most likely that the name is not activatable), let's print to stdout so it
gets paged. E.g. 'busctl tree org.freedesktop.systemd1 org.freedesktop.systemd2'
gives a nicely usable output.