tree-wide: generate SD_BUS_ERROR_INVALID_ARGS when we get invalid arguments on bus...

tree-wide: generate SD_BUS_ERROR_INVALID_ARGS when we get invalid arguments on bus calls

Let's make sure that when we return a D-Bus error, we return a native
one, if we generate it ourselves, and use errno-based error
synthetization only if we received an errno ourselves. Yes, this makes
things slightly longer, but is highly misleading as we propagate D-Bus
errors, and not errnos to the client.