Issue an error message when fts_read fails. Fixes bug #39324.

* find/ftsfind.c (find): when fts_read fails, issue an error
message, set the exit status to zero and stop.  Previously the
program would just stop (i.e. it failed to distinguish "done" from
"failed").
* find/find.1 (-exec): explain that on failure, some pending
command launches may not happen.  The Texinfo documentation
already pointed this out, so that didn't need to be changed.
* NEWS: Mention this bugfix.