For each known index idempotency problem, return a unique error to initial sync so that it can discern them and restart initial sync without failing tests. This will permit initial sync to detect unknown idempotency problems and fail tests rather than silently hide the problems.
There are a number of additional index operations that should be idempotent and currently not handled by the improvements in
Applying the following operations on a collection before and after dropping it will cause initial sync to restart. They were found by going through the various checks in IndexCatalog::_doesSpecConflictWithExisting
1. creating indexes with different specs but the same name.
2. creating text indexes with different specs.
3. having more than 64 indexes combined, before and after dropping a collection.