pytest: create proper mock failures.

We actually produce an invalid JSON error at the moment: bitcoin-cli
complains "JSON value is not an integer as expected" rather than returning
the given error.  Make our error a valid JSON RPC error.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/btcproxy.py

@@ -56,6 +56,7 @@ class BitcoinRpcProxy(object):
         except JSONRPCError as e:
             reply = {
                 "error": e.error,
+                "code": -32603,
                 "id": r['id']
             }
         self.request_count += 1