We fetch missing docs in initial sync because MMAP can miss docs in collection scan if they are updated. However, we should not fetch the doc for any errors other than the document is not found.

If an insert fails because of idempotency issues, inserting the latest version of doc may run into a duplicated _id error. This particular issue can be fixed by SERVER-26202 so that the insert won't fail, but adding missing docs should be limited to document not found error.