[enhancement] change behavior when extracting target is not found

[sasezaki]

I will change behavior about extracting target is not found.
No longer throw exception.
Because, target is not found is expectable - like db query behavior.

Migration Notes & WARNING

• this change can't detect that 'false' is supplied by "extracting target is not found" or filters into it.
• You should check scraping results has not false.

try {
   $scraper->process('//bad_target', 'key', 'html')
   ->scrape($url);
} catch (Exception $e) {}

↓

   $result = $scraper->process('//bad_target', 'key', 'html')
   ->scrape($url);

  if (!isset($result['key'])) {
    var_dump('error!');
  }

ToDo

• notes to README
• storing or logging each not found target (NEXT MAJOR VERSION)