🖊️ Add new indent error messages for level 8

[boryanagoncharenko]

Fixes #3270
Level 8 introduces indentation and allows single blocks while later levels allow nested blocks. Therefore, level 8 requires slightly different error messages.

Please consider whether the error messages could have better text.

How to test
Run Hedy locally and run the following snippets in level 8. The error messages should be meaningful and should not mention 'block in a block':

print 'test'
    print 'Hello folks'

repeat 5 times
print 'Hello folks'

repeat 5 times
    print 'test'
        print 'Hello folks'

repeat 5 times
    print 'test'
      print 'Hello folks'

repeat 5 times
    print 'test'
  print 'Hello folks'

[jpelay]

Everything works!!!

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).