src/update_handler: slightly improved error output for open_slot_device()
src/update_handler: slightly improved error output for open_slot_device()

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

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