Merge bitcoin/bitcoin#25106: rpc: dumptxoutset: check `fopen` return code

9feb887082be911a8342f8090af4dca3db76db9b rpc: check `fopen` return code in dumptxoutset (Sebastian Falbesoner)

Pull request description:

  This change improves the usability of the `dumptxoutset` RPC in two ways, in the case that an invalid path is passed:
  1. return from the RPC immediately, rather then when the file is first tried to be written (which is _after_ calculating the UTXO set hash)
  2. return a proper return code and error message instead of the cryptic message that appears on master currently (see below)

  master branch:
  (error message appears after several minutes on my machine)
  ```
  $ ./src/bitcoin-cli dumptxoutset /invalid/path
  error code: -1
  error message:
  CAutoFile::operator<<: file handle is nullptr: unspecified iostream_category error
  ```

  PR branch:
  (error message appears immediately)
  ```
  $ ./src/bitcoin-cli dumptxoutset /invalid/path
  error code: -8
  error message:
  Couldn't open file /invalid/path.incomplete for writing.
  ```

ACKs for top commit:
  w0xlt:
    Code Review ACK https://github.com/bitcoin/bitcoin/pull/25106/commits/9feb887082be911a8342f8090af4dca3db76db9b

Tree-SHA512: e8695a7e86f26cc3b086d6bc6888388061f1dee439f76409b3ee11d35032bfd9cfa5349b728cd7f45bcffd999ecf9a6a991be172ce587b9b14503d9916b6e984

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