Pyright script checker: better error if pyright fails to run

[Tom-Willemsen]

Pyright keeps a cache directory at c:\Users\spudulike\.cache. This can get corrupted, if it does get corrupted the error looks like:

Documented for now at https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/genie_python-Troubleshooting#pyright-completely-fails-to-run (resolution is just delete the cache and try again), but we should catch non-json output from pyright in a better way and propagate a more descriptive error out to the user.