message: Return ALREADY_EXISTS instead of UNAVAILABLE error when profiling is already active
  Makes C++ code consistent with python APIs.
diff: @@ -118,7 +118,7 @@ ProfilerSession::ProfilerSession(ProfileOptions options)
  options_(std::move(options)) {
  #if !defined(IS_MOBILE_PLATFORM)
  if (!active_) {
  - status_ = tensorflow::Status(error::UNAVAILABLE,
  + status_ = tensorflow::Status(error::ALREADY_EXISTS,
  "Another profiler session is active.");
  return;
  }
  