Display error message for non-existent chapters

Checking whether chapter_number is in chapter_urls before passing
possibly non-existent key to chapter_urls. If the chapter_number is not
an existing key in chapter_urls, display an error message instead of
crashing.
Display error message if manga does not exist

If the manga does not exist, the invalid URL will redirect to a search
page. If the page is a search page, display an error message and the top
ten most viewed simlar mangas.