Require message_id for Channel#load_message

[mattantonelli]

Summary

Ensures the message_id parameter of Channel#load_message is not nil since we cannot load the message without it. Calling the method with a nil value will now return a more helpful ArgumentError instead of a Discordrb::Errors::UnknownError.

Added

Added a check to Channel#load_message to ensure the message_id parameter is not nil.

Before

bot.channel(12345).message(nil)

[ERROR : main @ 2021-08-13 17:40:53.860] 404: Not Found
Discordrb::Errors::UnknownError: 404: Not Found

After

bot.channel(12345).message(nil)

ArgumentError: message_id cannot be nil

[swarley]

Thanks!