Move error-handling into its own file

[ashuping]

Error-handling code is currently in the main hector.py file. This clutters up the main file. The code should instead be moved into its own file, `mod.core.errors', and loaded into the bot like the other core modules.

This issue will be resolved when all error-handling code (i.e. generating embeds, handling error messages, and updating the messages in response to reactions) is done in mod.core.errors instead of in hector.py