message: rpc: Check that left section is not multiline
diff: @@ -398,6 +398,9 @@ struct Sections {
  std::string ret;
  const size_t pad = m_max_pad + 4;
  for (const auto& s : m_sections) {
  + // The left part of a section is assumed to be a single line, usually it is the name of the JSON struct or a
  + // brace like {, }, [, or ]
  + CHECK_NONFATAL(s.m_left.find('\n') == std::string::npos);
  if (s.m_right.empty()) {
  ret += s.m_left;
  ret += "\n";
  