Revert "bus-proxyd: improve compatibility with dbus-1"

This reverts commit b0f84d4d7832659f2216bda7a7cdf51f5e79c6eb.

get_creds_by_name() already translate the error nicely, we just need to
make use of it.
bus-proxy: let's make use of the translated errors get_creds_by_name() provides us with