Try load RPC wallet on start-up if it's not loaded

[kristapsk]

Resolves #812.

[PulpCattel]

@kristapsk

After this PR, if using 0.21+ version of Bitcoin Core, scripts like snicker-finder.py and ob-watcher.py will crash at start when using an empty rpc_wallet_file. Those scripts do not need a wallet though.
IIUC, it's because Bitcoin Core does not create a default wallet anymore.

(jmvenv) user@host:~/Dev/projects/joinmarket-clientserver/scripts/snicker$ python3 snicker-finder.py -j 682751
User data location: /home/user/.joinmarket/
Traceback (most recent call last):
  File "snicker-finder.py", line 115, in <module>
    main()
  File "snicker-finder.py", line 76, in main
    load_program_config(config_path=options.datadir)
  File "/home/user/Dev/projects/joinmarket-clientserver/jmclient/jmclient/configure.py", line 627, in load_program_config
    global_singleton.config)
  File "/home/user/Dev/projects/joinmarket-clientserver/jmclient/jmclient/configure.py", line 714, in get_blockchain_interface_instance
    rpc_wallet_file)
  File "/home/user/Dev/projects/joinmarket-clientserver/jmclient/jmclient/jsonrpc.py", line 71, in __init__
    self.call("loadwallet", [wallet_name])
  File "/home/user/Dev/projects/joinmarket-clientserver/jmclient/jmclient/jsonrpc.py", line 169, in call
    raise JsonRpcError(response["error"])
jmclient.jsonrpc.JsonRpcError: {'code': -18, 'message': "Wallet file verification failed. Failed to load database path '/home/user/.bitcoin'. Data is not in recognized format."}

I encountered this because I have an installation of JM used solely as a "toolbox", so I don't have any Bitcoin Core wallet but I still connect to RPC.

[AdamISZ]

I had the same issue in testing; as I understand it, it's now essential to have rpc_wallet_file filled in in your joinmarket.cfg, since 0.21, because of this. Am I correct?

[PulpCattel]

Seems so. BTW, I think it's generally a good thing, right? It was a best practice anyway.
It's a bit annoying and counter intuitive only when we are talking about those utility scripts (and also it should probably be clarified in the doc, that for 0.21+ is required and not just recommended).

e.g., snicker-finder says:

This script does NOT require a wallet, but it does require
a connection to Core, so does not work with no-blockchain.

It can be read as in "JM wallet", and from this POV it's still correct, but maybe it should be clarified; a user will not expect a dummy Bitcoin Core wallet to be required, especially after reading the above description.

[AdamISZ]

Yes, absolutely. I didn't anticipate that at all.