Improve ScriptText validation error messages and prevent circular references

[glx22]

Closes #10542

Motivation / Problem

As mentioned in #10542, ScriptText validation error messages are quite generic.
Also while testing the new messages I had a silly test idea which triggered a nice infinite recursion.

Description

Give more details in the error message to help script authors.
Introduce a mechanism to prevent circular references in ScriptText.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker