STITCH-2248 Swift SDK: Split [dataSync] errors between recoverable and irrecovera…

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…ble errors

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The user's error delegate would receive these errors:

func on(error: DataSynchronizerError, forDocumentId documentId: BSONValue?) {
        switch error {
        case .decodingError(_):
            // if I receive this error, the message will contain information
            // telling me that I've messed with a documents version info to the point
            // that it can no longer be read or decoded.
            // So if that happens, I could fix the error, and then unpause
            // the document.
            break
        case .unsupportedProtocolVersion(_):
            // if I receive this error, the message will contain information
            // telling me that I'm using an unsupported protocol version.
            break
        case .documentDoesNotExist(_):
            // if I receive this error, chances are I've done something in my
            // own application that is racy.
            break
        case .mongoDBError(_, _):
            // if I receive this error, there will be more information within the
            // secondary error as to what I've done
            break
        case .resolutionError(_):
            // if I received this error, I would look to my conflict
            // resolver to see what error could be surfacing here
            break
        case .fatalError(let msg):
            // This error tells me that we've met with a terrible fate.
            // The user is likely trying to use the application on a system
            // that embedded mongo does not support.
            fatalError(msg.localizedDescription)
        }
    }

It's generally so the user could debug. Documents that have been paused will not be synced on. So a user would have to fix their issues, and then unpause the document manually, which we expose methods for.