Logic and performance fixes

[3.0.3] - 2/17/20

Added

• invalid_chars attribute for NotFoundError when applicable
• url attribute for all errors that have requested a URL
• str(error) will return the message attribute of the error.

Changed

• The error attribute for all errors have been renamed to message
• For BrawlAPI: get_leaderboard split up into get_player_leaderboard, get_club_leaderboard, and get_brawler_leaderboard
• For the official API: get_rankings split up into get_player_rankings, get_club_rankings, and get_brawler_rankings