Better error message when chapter or section name contains invalid character

[fingolfin]

I wrote some documentation and dared to put a colon into a chapter title. This triggered the following error:

Generating documentation in Directory("/PATH/TO/doc/")
Error, <name> must not contain '\' or ':' called from
Filename( arg[1], arg[2] ) called from
AUTODOC_OutputTextFile( path_to_xmlfiles, filename ) called from
WriteDocumentation( i, stream, path_to_xmlfiles ); called from
WriteDocumentation( tree, doc_dir ); called from
<function "AutoDoc">( <arguments> )
 called from read-eval loop at line 21 of makedoc.g
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

That is not very helpful. I had written a big chunk of stuff, and part of it contained a backslash. After removing that, I still got errors, and it took me some time that I should search for a colon...

I think we should check chapter and section names the moment we read them in, and immediately print an error at that point, if necessary -- including filename, line and the precise string that was read...

[sebasguts]

I think we could discuss this together with #78. In general, better error messages are something I'm working on, but I still don't have anything working.

[sebasguts]

Also #79

[fingolfin]

Note that with my changes in a4de357 it should now be relatively (?) easy to implement a better error message. Basically, we need a function which validates a given any chapter / section / subsection title we just read. And if the title contains characters outside a certain predefined set, the function raises an error, pointing out the file and line with the "bad" title.

[fingolfin]

Alternatively, we could simply remove this error, by following the discussion in #78