fts(3) can run (albeit more slowly and imposing the {PATH_MAX} limit) when
the current directory cannot be opened. Therefore, do not make a failure to
open the current directory (for returning to it later in -exec) fatal.
If -execdir or -delete are used, the expectation is that fts(3) will use
chdir to avoid race conditions (except for -execdir with -L). Do not break
this expectation any more than it already is by still failing if the current
directory cannot be opened.