Merge bitcoin/bitcoin#30267: assumeutxo: Check snapshot base block is not in invalid chain

2f9bde69f45c7a9fdcf0c65f9e1305391a6f1f28 test: Remove unnecessary restart in assumeutxo test (Fabian Jahr)
19ce3d407ef546fa50d18b2ffbd67b7417797064 assumeutxo: Check snapshot base block is not marked invalid (Fabian Jahr)
80315c011863d69e7785673283e4c9033fbcd5ac refactor: Move early loadtxoutset checks into ActiveSnapshot (Fabian Jahr)

Pull request description:

  This was discovered in a discussion in #29996

  If the base block of the snapshot is marked invalid or part of an invalid chain, we currently still load the snapshot and get stuck in a weird state where we have the snapshot chainstate but it will never connect to our valid chain.

  While this scenario is highly unlikely to occur on mainnet, it still seems good to prevent this inconsistent state.

  The behavior change described above is in the second commit.

  The first commit refactors the early checks in the `loadtxoutset` RPC by moving them into `ActivateSnapshot()` in order to have the chance to cover them by unit tests in the future and have a more consistent interface. Previously checks were spread out between `rpc/blockchain.cpp` and `validation.cpp`. In order to be able to return the error message to users of the RPC, the return type of `ActivateSnapshot()` is changed from `bool` to `util::Result`.

  The third commit removes an unnecessary restart introduced in #29428.

ACKs for top commit:
  mzumsande:
    re-ACK 2f9bde6
  alfonsoromanz:
    Re-ACK 2f9bde69f45c7a9fdcf0c65f9e1305391a6f1f28. The RPC code looks much cleaner after the refactor. Also, it seems very useful to get the error message in the RPC response rather than having to rely on the logs in some scenarios if you are an RPC user.
  achow101:
    ACK 2f9bde69f45c7a9fdcf0c65f9e1305391a6f1f28

Tree-SHA512: 5328dd88c3c7be3f1be97c9eef52ac3666c27188c30a798b3e949f3ffcb83be075127c107e4046f7f39f961a79911ea3d61b61f3c11e451b3e4c541c264eeed4

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