Merge bitcoin/bitcoin#25239: wallet: 'CommitTransaction', remove extra wtx lookup and add exception for db write error

57fb37c27599fc865f20b42a27bb9c227f384de3 wallet: CommitTransaction, remove extra wtx lookup and add exception for a possible db write error. (furszy)

Pull request description:

  Two points for `CWallet::CommitTransaction`:

  1) The extra wtx lookup:
      As we are calling to `AddToWallet` first, which returns the recently added/updated wtx pointer, there is no need to look up the wtx again few lines later. We can just use it.

  2) The db write error:
      `AddToWallet` can only return a nullptr if the db write fails, which inside `CommitTransaction` translates to an exception throw cause. We expect everywhere that `CommitTransaction` always succeed.

  ------------------------------------------------

  Extra note:
  This finding generated another working path for me :)
   It starts with the following question: why are we returning a nullptr from `AddToWallet` if the db write failed without removing the recently added transaction from the wallet's map?..
   Can led to a wallet invalid state where the inputs of this new transaction are not marked dirty, while the transaction that spends them still exist on the in-memory wallet tx map.
  -- I'm writing it here to gather some feedback first and not forget it, will create a follow-up PR in the coming days 🚜 --

ACKs for top commit:
  achow101:
    ACK 57fb37c27599fc865f20b42a27bb9c227f384de3
  jonatack:
    ACK 57fb37c
  ryanofsky:
    Code review ACK 57fb37c27599fc865f20b42a27bb9c227f384de3. Seems like a clear improvement. Better to fail earlier with a better error message if the failure is going to happen anyway

Tree-SHA512: 80e59c01852cfbbc70a5de1a1c2c59b5e572f9eaa08c2175112cb515256e63fa04c7942f92a513b620d6b06e66392029ebe8902287c456efdbee58a7a5ae42da

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