bus: rework sd_bus_error APIs

All calls that set a sd_bus_error structure will now return the same
error converted to a negative errno. This may be used as syntactic sugar
to return from a function and setting a bus_error structure in one go.

Also, translate all Linux Exyz (EIO, EINVAL, EUCLEAN, EPIPE, ...)
automatically into counterparts in the (new) "Posix.Error." namespace.

If we fail to allocate memory for the components of a sd_bus_error
automatically reset it to an OOM error which we always can write.