src/main: use g_printerr() for errors instead of setting message
src/main: use g_printerr() for errors instead of setting message

If setting 'message' variable, this will be printed as non-error message
afterwards. Thus we should use the 'message' variable for error strings
only

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

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