Why don't we need handleReduxMigrationFailure here?

• In D13889, original unshimClientDB was renamed to legacyUnshimClientDB, and new unshimClientDB was introduced. The former was used for legacy migrations.
• There is some context in D11928 as to why handleMigrationFailure was introduced as a parameter to the unshim func. But I guess it's only for native, and web has never had this param.

Then in https://phab.comm.dev/D13889 handleReduxMigrationFailure was replaced by rethrowing an exception.

In https://phab.comm.dev/D13889?id=45703 @ashoat did these changes to the unshim func after I failed to do the migration (see the diff history). I didn't have much knowledge about unshimming and migrations and I didn't argue with these changes. Unfortunately, I have no more context beyond that.
Just note that migration I introduced there was native-only and it was no-op for web, so the unshim func wasn't really tested.

Anyway, since the unshim func on web doesn't take the handleMigrationFailure param, this diff is okay and any fixes/adjustment to the unshimming should probably be done separately.

It looks like native's unshimClientDB is different since it doesn't throw an exception. It would probably be safe to change it to match web given that both use createAsyncMigrate, but I suppose that's outside the scope of this diff, and probably not the best thing to prioritize right now