fix: defensive RPC response handling

[al-munazzim]

Problem

Fixes #2506. When Bitcoin Core (or an RPC proxy) returns a non-standard JSON-RPC response, the current error handling in BitcoinRPC.__getattr__ can crash with KeyError:

• r['error']['message'] assumes error is always a dict with a message key
• r['result'] assumes the response always contains a result key
• multi()[0] assumes the response is always a list

Root Cause

The JSON-RPC spec says error should be an object with code and message, but in practice:

• Some proxies return error as a plain string
• Some implementations omit the message field
• Some return a single object instead of a batch array
• Connection issues can produce responses missing result entirely

Fix

1. __getattr__ / fn(): Check if error is a dict before accessing .get('message'); fall back to str(error). Also check for missing result key explicitly.
2. multi(): Wrap single-dict responses in a list so [0] indexing always works.

Testing

Verified manually with mocked responses covering all 4 edge cases:

• String error field → proper RpcError
• Missing result key → proper RpcError
• Single dict (non-batch) response → works correctly
• Error dict without message key → proper RpcError with fallback text

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