This fixes the error reporting if you make two attempts
to activate a service that cannot be activated due to an
error that is reported synchronously, such as a system
service with no User= line in its .service file.

This is easy to reproduce with the gdbus(1) tool, which
sends an Introspect call in addition to the one you asked
it to. If you try to activate a service using

gdbus call --session -d com.example.FailToActivate \
    -o / -m org.freedesktop.DBus.Peer.Ping

then gdbus will actually send two method calls: one
Introspect, and one Ping. The Introspect gets the correct
error reply, but when dbus-daemon enters
bus_activation_activate_service() for the Ping call, it
sees that there is a pending activation and does an
early-return. The pending activation does not finish
until the timeout is reached.

A couple of error cases handled this correctly, but the
majority did not; make them all go into the same code path.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=92200


Reviewed-by: Thiago Macieira <thiago@kde.org>