Since we now write a (partial) view object of the exported branches to
disk (since 7904474320), we can safely skip exporting some
branches. We already skip conflicted branches. This commit makes us
also skip branches that we fail to write to the backing Git repo,
instead of failing the whole operation (after possibly updating some
Git refs).
I made the `export_refs()` function return the branches that
failed. We should probably make that a struct later and have a
separate field for branches that we skipped due to conflicts.
Closes#493.
4a03b94d65