genie_python: Validate block names in load script

As a user I would like genie python to validate on load_script that when I run the script what I am setting/getting are valid block names so that I don't make typos. MARI has already implemented something like this themselves using the simulation mode created in #5480 (e.g https://github.com/ISISComputingGroup/ibex_user_manual/wiki/Simulating-Scripts). Once done discuss with MARI as to whether they can remove their code.

### Acceptance Criteria

* When I load a script that tries to cset/cget a non-existent block I get an error
* This should be optional and off by default