에러 페이지를 에러 컨트롤러에서 다루도록 변경

에러 페이지더라도 같은 레이아웃을 사용하게 되서 상단바가 사라지지 않음.
우선은 404, 500 에러에 대해서만 처리
Merge pull request #33 from seapy/master

에러 페이지를 에러 컨트롤러에서 다루도록 변경