rust-status: don't bubble up os errors, translate them to bad matches
In the rare cases when either the OS/filesystem throws an error on an otherwise
valid action, or because a path is not representable on the filesystem, or
because of concurrent actions in the filesystem, we want to warn the user about
said path instead of bubbling up the error, causing an exception to be raised
in the Python layer.
Differential Revision: https://phab.mercurial-scm.org/D9320