We define custom nicks for each error enum, matching the DBus error entry that
we want to have with each of them.

Lets operations specify that they cannot be run currently, but that the caller
may retry later. Of course, it depends on the caller to really honour the
retry-later suggestion.

To be returned when we try to send an AT command to a modem, but there is no
usable AT port available, as the primary port is connected (hence the name)
and there is no secondary port.

Errors that will/may be reported via DBus are defined in the public interface
header.