Fix pruning of partial commits

Fix pruning of partial commits

If we have a partial commit it is not an error for a dirmeta to be
missing (in fact, that is likely), so instead of returning a not-found
error from ostree_repo_traverse_commit() we ignore the error and
continue.

In particular, this means we don't stop early at the first
missing dirmeta, which previously caused ostree_repo_prune() to
thing the dirmetas after that to be unreached and thus purged.

Also, we remove the special casing in ostree_repo_prune() to
not report errors for commitpartial, because these should not
be reported anymore.

This fixes https://github.com/ostreedev/ostree/issues/541

Closes: #542
Approved by: cgwalters