message: rpc: Prefix rpcwaittimeout error with details on its nature
  As proposed by the error message is now prefixed with the
  "timeout on transient error:" prefix, to explain why the error is
  suddenly considered terminal.
diff: @@ -809,12 +809,12 @@ static UniValue ConnectAndCallRPC(BaseRequestHandler* rh, const std::string& str
  }
  }
  break; // Connection succeeded, no need to retry.
  - } catch (const CConnectionFailed&) {
  + } catch (const CConnectionFailed& e) {
  const int64_t now = GetTime<std::chrono::seconds>().count();
  if (fWait && (timeout <= 0 || now < deadline)) {
  UninterruptibleSleep(std::chrono::seconds{1});
  } else {
  - throw;
  + throw CConnectionFailed(strprintf("timeout on transient error: %s", e.what()));
  }
  }
  } while (fWait);
  