bus-proxyd: improve compatibility with dbus-1

'GetConnectionUnixProcessID', 'GetConnectionUnixUser' and
'GetConnectionSELinuxSecurityContext' methods should return
'NameHasNoOwner' error (if chosen name is not available on bus)
with more detailed description - like dbus-1:

Could not get PID of name 'org.freedesktop.test': no such name.
Could not get UID of name 'org.freedesktop.test': no such name.
Could not get security context of name 'org.freedesktop.test': no such name.

Otherwise we have only laconic message without proper dbus error:

Error System.Error.ENXIO: No such device or address