operate with extern.values instead of iterating over extern.items

to make it clear we don't use the keys (name)
move _categorize_errors docstring to _process_errors

we've consolidate message creation there, so that's where that docstring belongs
fix typo in docstring
make indent of messages 4 spaces instead of 3
fix FIXMEs in BrokenLinkage docstring
modify SlotObject class docstring

with notes about the __init__ function of subclasses