src/main: check positional args first
src/main: check positional args first

Not providing one of the required positional args should be a more
severe error than not providing an argument.

Thus we check this first now.

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

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