Implemented generic command error handling

Also:
- Removed emotes from some of the error messages.
- Changed how emotes were placed into the leaderboard slightly
- Moved around TODO strings into proper files and added more.
- Corrected main.py cog loading references.
- Improved ContestBot.reject with message references and used built-in
delete_after keyword argument.
- Minor docstring/light formatting

bot/models.py

@@ -23,9 +23,8 @@ logger.setLevel(constants.LOGGING_LEVEL)
 
 Base = declarative_base()
 
-
-# TODO: Setup and test basic automatic migration.
-
+# TODO: Contest names
+# TODO: Refactor Period into Contest (major)
 
 class PeriodStates(enum.Enum):
     """