Fix exports query with failed RPCs

When the RPC to a node has failed (for whatever reason), we currently
return _FS_UNAVAIL, which means "this node doesn't support
exports". However, that is misleading: all nodes support exports, but
we failed to list them at the moment, so _FS_NODATA (temporary
failure) is more appropriate here.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>