Bug: if one .gd file has a `@Chapter` and `@Section` but another doesn't an unhelpful error is raised

[fingolfin]

We edited a package to add @Chapter and @Section to one .gd file (before there was only an auto-generated chapter, with auto-generated sections.

This lead to a really unhelpful error:

Generating documentation in Directory("/Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/N\
autyTracesInterface/doc/")
Error, Record Element: '<rec>.Chapter_NautyTracesInterface_automatic_generated_documentation_Sec\
tion_NautyTracesInterface_automatic_generated_documentation_of_methods' must have an assi\
gned value in
  Add( tree!.nodes_by_label.(context), group
 ); at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/DocumentationTree.gi:\
290 called from
DocumentationGroup( tree, GroupName( node ), chapter_info
 ) at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/DocumentationTree.gi:3\
29 called from
Add( tree, man_item
 ); at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/Parser.gi:216 called from
add_man_item(
 ); at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/Parser.gi:344 called from
Scan_for_Declaration_part(
 ) at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/Parser.gi:897 called from
AutoDoc_Parser_ReadFiles( files_to_scan, tree, default_chapter_record
 ); at /Users/mhorn/Projekte/GAP/repos/pkg/gap-packages/AutoDoc/gap/AutoDocMainFunction.g\
i:344 called from
...  at makedoc.g:10
type 'quit;' to quit to outer loop

This doesn't help at all in figuring out what is wrong, and where it is wrong.

• there should be a helpful error
• there should aso be an indication which file we are parsing and which line (just as GAPDoc outputs)
• and in this case, it is not even clear to me why there is an error in the first place, so perhaps we can also make it not-error here. But that's a separate topic.