Make pyivastest family of scripts fail with an error when no items are found for given modes

When running runIvasCodec.py using a config json file which contains file paths that do not exist on your system leads to an infinite loop. E.g. scripts/config/ci_linux.json expects all input files to be in /usr/local/testv/ which is is the case for the CI runners, but probably not for your local machine. The script then prints

Start running 891 modes with 0 encs and 0 decs

and halts forever. To prevent this from stalling misconfigured runners in ci and to make debugging CI easier, a fix is proposed taht instead raises an error.