rust/bwrap: log fusermount -u errors to stderr

I believe cosa is hitting an error in this code path, but we can't
inspect the error because there's no journald running in supermin:

coreos/coreos-assembler#3848

We don't gain much by printing this error to the journal since it
shouldn't be very common. Print it on stderr instead where it's more
likely to have visibility.

rust/src/bwrap.rs

@@ -136,7 +136,7 @@ impl Drop for RoFilesMount {
             })
             .err()
             .map(|e| {
-                systemd::journal::print(4, &format!("{}", e));
+                eprintln!("{}", e);
             })
             .is_none();
         if !success {