As part of the WT-10716 code review, __tree_walk_internal now checks whether prefetching content would be helpful, and if so, does the prefetching work. This work can fail, and it's currently wrapped in a WT_ERR, so the error would be bubbled back up to the user.

Swallowing errors for optimisations is something we do in other parts of the code - for example, in the POSIX filesystem, we try to mmap files, but if it fails we swallow it and mark the handle as non-mappable.

Prefetch is ultimately an optimisation, and we should evaluate how to handle these errors.