[Request] Error reporting for broken GCODE scripts

Is your feature request related to a problem? Please describe.

I have just had an extremely annoying situation where my prints repeatedly failed because I had a plugin referenced in the gcode "before print job" section that I'd uninstalled, and my printer was no longer bed levelling because the gcode script wasn't executing. It took me ages to figure this out.

Errors in custom gcode really need to be flagged up somewhere - personally I'd expect the job to fail and a dialog to pop up. Could things like broken templates be flagged up at the time you attempt to save them in settings?

Originally posted by @Jonty in https://github.com/OctoPrint/OctoPrint/issues/2414#issuecomment-734491009

Describe the solution you'd like

Try to prerender gcode scripts with an example context upon save and check if an error gets produced, if so refuse to save and show a warning to the user. Maybe a testing endpoint is needed that can be called from the frontend?

Describe alternatives you've considered

None

Additional context

#2414

No files or tests are named. Start by tracing the settings flow for custom GCODE and the existing script/template rendering path, then review the related discussion in #2414. Done means invalid scripts are detected on save, the user receives a warning, and any needed frontend testing endpoint is defined.