Silence expected error logs

Summary:
When an app with sonar is run for the first time, the necessary files don't exist.
This is expected, so don't output an error, only error if they do exist but there's some other problem.

Reviewed By: emilsjolander

Differential Revision: D8350995

fbshipit-source-id: ff0a4f0e7a73848f6172c6108a0caee6efb43553

xplat/Sonar/SonarWebSocketImpl.cpp

@@ -42,6 +42,8 @@ static constexpr int insecurePort = 8089;
 namespace facebook {
 namespace sonar {
 
+bool fileExists(std::string fileName);
+
 class ConnectionEvents : public rsocket::RSocketConnectionEvents {
  private:
   SonarWebSocketImpl* websocket_;
@@ -249,6 +251,9 @@ void SonarWebSocketImpl::requestSignedCertFromSonar() {
 }
 
 std::string SonarWebSocketImpl::loadStringFromFile(std::string fileName) {
+  if (!fileExists(fileName)) {
+    return "";
+  }
   std::stringstream buffer;
   std::ifstream stream;
   std::string line;
@@ -283,5 +288,10 @@ bool SonarWebSocketImpl::ensureSonarDirExists() {
   }
 }
 
+bool fileExists(std::string fileName) {
+  struct stat buffer;
+  return stat(fileName.c_str(), &buffer) == 0;
+}
+
 } // namespace sonar
 } // namespace facebook