chore: remove SplitRecordError during get

[RolandSherwin]

• in case of a split spend, they are collected and if it is a double spend we return NetworkError::DoubleSpendAttempt
• in case of a split register, they are verified, merged and returned as a record.

so we don't have to expose the SplitRecordError from sn_networking. Use a private error enum to handle the low level errors and make the public error enum clean.