_parser_common_syndication_sanitize_cache should throw an error when the cache is not writeable [#723590]

It took me some time to debug it the errors I got when trying to save a feed. logging it here for the records:

# warning: fopen(/c7add89e4e38ae2bc5ac48059c4907bb) [function.fopen]: failed to open stream: Permission denied in .../sites/all/modules/feedapi/parser_common_syndication/parser_common_syndication.inc on line 69.
# warning: fwrite(): supplied argument is not a valid stream resource in .../sites/all/modules/feedapi/parser_common_syndication/parser_common_syndication.inc on line 70.
# warning: fclose(): supplied argument is not a valid stream resource in .../sites/all/modules/feedapi/parser_common_syndication/parser_common_syndication.inc on line 71.

It comes the from following function:

function _parser_common_syndication_sanitize_cache() {
  $cache_location = file_directory_path() .'/parser_common_syndication_cache';
  if (!is_writeable($cache_location) || !is_dir($cache_location)) {
    $cache_location = file_create_path($cache_location);
    if (!file_exists($cache_location) && is_writable(file_directory_path())) {
      mkdir($cache_location);
    }
    if (!is_writeable($cache_location)) {
      return FALSE;
    }
  }
  return $cache_location;
}

in my case, the directory sites/madrc/files/parser_common_syndication_cache existed, but the cache path parser_common_syndication_cache/c7add89e4e38ae2bc5ac48059c4907bbwas not writeable, so the function above returned FALSE. Instead it should either throw an drupal_message error or the callers should be able to deal with the case where FALSE is returned. Currently, it silently fails and Drupal tries to write to /c7add89e4e38ae2bc5ac48059c4907bb.