test/manifest.c: enhance testing for return codes and errors
test/manifest.c: enhance testing for return codes and errors

Test for potential errors and error codes explicitly where not done yet.

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

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