Migrate on_command_error pin

[Robin5605]

As part of the migration of the #discord-bots pinned content from discord to the site, this PR migrates the pin regarding on_command_error of the discord.py library "eating" (silencing) unhandled errors.

This PR also adds a few extra items, including:

• General markdown improvements, formatting
• Fixing a few inaccuracies (e.g generic commands.BagArgument has been changed to commands.MemberNotFound
• An elif statement handling another error, showing users that they can handle any number of errors they want to
• Showing how to use specific properties of the errors commands.MemberNotFound.argument, commands.MissingRequiredArgument.param
• Implementing the on_command_error function with and without a bot subclass, as the function in itself does not do anything, and needs to be bound to the bot somehow

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[coveralls]

Coverage remained the same at 100.0% when pulling 0ff39f6cef7d889cc2e14918d65ffaaf05efec6f on Robin5605:discordpy-error-handling into 4ce9f301a7c74e50f23cb2ddaf22536a4af31ac4 on python-discord:main.