Introduce dbusErrors module

Quoting the main commit:

dbusErrors: Add utility module for handling remote errors

We sometimes return custom D-Bus errors. Right now those errors are ad-hoc, which means receivers don't have a proper way of checking whether a thrown GLib.Error corresponds to a particular remote error.

In order to change that, we should define proper error enums and domains, and register them with GDBus, so that there is an automatic mapping between D-Bus errors and GLib.Errors.

The errors added in this MR are more or less cosmetic. The real
use case is in !2976, which currently abuses the `message` field
for propagating machine-readable error information.