src/bootchooser: custom_backend_get: do not return data on failure
src/bootchooser: custom_backend_get: do not return data on failure

Check the exit code of custom bootloader executable before obtaining
(and returning) the data.

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

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