Fix crash when lyrics not yet released on Genius
Fix crash when lyrics not yet released on Genius

There are some tracks on Genius whose lyrics are yet to be released.
When the tool previously attempted to scrape such webpages, it
resulted in a crash.
The tool will now raise an exception; LyricNotFoundError when such a
track is encountered, which is then handled internally by the tool.

Example of one such track on Genius:
https://genius.com/The-federal-empire-good-man-lyrics

Fixes #647.

Ritiek/Spotify-Downloader