src/service: more targeted error message when loosing existing connection
src/service: more targeted error message when loosing existing connection

Up to now, when the D-Bus disappeared, RAUC exited with:

| Connection to the bus can't be made for de.pengutronix.rauc

This is quite misleading as the connection was actually present before
but was cut off. Thus we differentiate these two cases by checking for
an already-set r_installer that implies r_on_bus_acquired was called
successfully before.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc