Merge bitcoin/bitcoin#22660: contrib: catch bitcoin-cli RPC call errors in getcoins.py

42dbd9025adc200012c103d2c091cf026f1d50b1 contrib: return non-zero status if getcoins.py errors (Sebastian Falbesoner)
8c203cf0e1b9558b54030c284ddf7706d64cdde2 contrib: catch bitcoin-cli RPC call errors in getcoins.py (Sebastian Falbesoner)
0eca5ebaced264d041de815316eaca8cf6fef92f contrib: refactor: introduce bitcoin-cli RPC call helper in getcoins.py (Sebastian Falbesoner)

Pull request description:

  This PR is based on #22565 ("[script] signet's getcoins.py improvements"), which should be reviewed first.

  The signet faucet script `contrib/signet/getcoins.py` currently issues bitcoin-cli RPC calls without catching errors -- the only case tackled is if there is no `bitcoin-cli` file found. Instead of crashing with a stack-trace on a failed RPC call, the changes in this PR aim to produce a more user-friendly output (see also https://github.com/bitcoin/bitcoin/pull/22565#discussion_r683754875). Additionally, in case of any error, a non-zero status is now returned (instead of 0, indicating success), which could be useful for other scripts taking use of signet faucet script.

  The most straight-forward way to test this is invoking the script without a `bitcoind` running on signet:

  PR22565 branch:
  ```
  $ ./contrib/signet/getcoins.py
  error: Could not connect to the server 127.0.0.1:8332

  Make sure the bitcoind server is running and that you are connecting to the correct RPC port.
  Traceback (most recent call last):
    File "./contrib/signet/getcoins.py", line 26, in <module>
      curr_signet_hash = subprocess.check_output([args.cmd] + args.bitcoin_cli_args + ['getblockhash', '1']).strip().decode()
    File "/usr/local/lib/python3.8/subprocess.py", line 415, in check_output
      return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
    File "/usr/local/lib/python3.8/subprocess.py", line 516, in run
      raise CalledProcessError(retcode, process.args,
  subprocess.CalledProcessError: Command '['bitcoin-cli', 'getblockhash', '1']' returned non-zero exit status 1.
  ```

  this PR branch:
  ```
  $ ./contrib/signet/getcoins.py
  error: Could not connect to the server 127.0.0.1:38332

  Make sure the bitcoind server is running and that you are connecting to the correct RPC port.
  -----
  Error while calling "bitcoin-cli -signet getblockhash 1" (see output above).
  ```

ACKs for top commit:
  kallewoof:
    Code ACK 42dbd9025adc200012c103d2c091cf026f1d50b1
  Zero-1729:
    tACK 42dbd90 🧪

Tree-SHA512: 912240a4ed03c87035e370602f4095c7ffe26806421bbbd6cf86588126f2310a01a6a61606e9e2918fb2c1a0debdd0ce768c69ba2e4b8e7750fa3474a56d01a0

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