fix `Failed to stat` error w/ LoadFromFile ngx#972

If you turn on LoadFromFile then 404s turn into errors in the log. These
shouldn't be errors, so don't log unless we see an errno which isn't ENOENT.

pagespeed/kernel/base/stdio_file_system.cc

@@ -423,11 +423,12 @@ bool StdioFileSystem::Stat(const StringPiece& path, struct stat* statbuf,
  const char* path_str = path_string.c_str();
  if (stat(path_str, statbuf) == 0) {
    return true;
  } else {
    handler->Message(kError, "Failed to stat %s: %s",
                     path_str, strerror(errno));
    return false;
  } else if (errno != ENOENT) {  // Not an error if file doesn't exist see #972.
    // https://github.com/pagespeed/ngx_pagespeed/issues/972
    handler->Message(kError, "Failed to stat %s: %s", path_str,
                     strerror(errno));
  }
  return false;
}

// TODO(abliss): there are some situations where this doesn't work