"Could not find link destinations" warning could be more precise

[sjakobi]

Example:

Warning: Lib: could not find link destinations for:
    foldl foldl' foldl1 foldr foldr1 Array

I think this warning could be improved in (at least) two ways:

• Show the names used in the source (in this case List.foldl etc)
• Give source locations (if available?!)