Do not write the index cache from an index

The new .debug_names reader will work by creating a cooked index from
.debug_names.  This patch updates cooked_index::maybe_write_index to
avoid writing the index in this case.

However, in order to do this in a clean way, the readers are changed
so that a nullptr result from index_for_writing means "cannot be
done", and then the error message is moved into write_dwarf_index
(where it historically lived).