Wallet: Refactor ReserveKeyFromKeyPool for safety #13252

ReserveKeyFromKeyPool's previous behaviour is to set nIndex to -1 if the keypool is empty, OR throw an exception for technical failures. Instead, we now return false if the keypool is empty, true if the operation succeeded.

This is to make failure more easily detectable by calling code.

Based on work by luke-jr, #9537

Incidental changes relative to that work:

• Return bool rather than throw, because this is a common failure mode, so its handling is control flow
• Extract use_split_keypool bool to make "keypool entry misclassified" case more straightforward
• throw rather than assert on keypool.vchPubKey.IsValid(), to simplify docs and create an opportunity for handling / testability

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