Add debug code to identify htmlify problems

[JJ]

The problem

When htmlify.p6 fails like here:

Writing per-syntax files ...
..............No such method 'contents' for invocant of type 'Any'
  in block  at htmlify.p6 line 934
  in sub write-kind at htmlify.p6 line 914
  in sub MAIN at htmlify.p6 line 228
  in block <unit> at htmlify.p6 line 981

it's impossible to know what's the problem. It's got something to do with indexes, but who knows...

Suggestions

Insert some debug code that says where's the problem. Ref #1823