Incorrect use of errcode_for_file_access in backend code

Hi all,

While looking at another patch for slot.c, I have noticed what looks
like incorrect use of errcode_for_file_access:
- errcode_for_file_access() is used with rmtree(), which makes no sense
as this comes from common/rmtree.c, and a warning already shows up using
%m.
- ERRCODE_DATA_CORRUPTED is not used to mention corrupted data, and
instead errcode_for_file_access() gets called.

Wouldn't something like the attached provide more adapted error
handling? That's mostly error handling beautification, so I would be
incline to just fix HEAD.

(I have noticed some inconsistent error string format in autoprewarm.c
on the way.)

Thoughts?
--
Michael