Instead of raising a whole variety of exceptions, they are now wrapped
into ExtensionError derived classes.
* ExtensionAlreadyLoaded
* Raised when an extension is already loaded in Bot.load_extension
* ExtensionNotLoaded
* Raised when an extension is not loaded, e.g. Bot.unload_extension
* NoEntryPointError
* Raised when an extension does not have a `setup` function.
* ExtensionFailed
* Raised when an extension's `setup` function fails.
* ExtensionNotFound
* Raised when an extension's module import fails.
This causes them to be raised from a new exception named
ArgumentParsingError with 3 children for ease with i18n. This is
technically a breaking change since it no longer derives from
BadArgument, though catching UserInputError will prevent this change
from affecting the user.
This assumes that a Converter class raising non-CommandError
is a programmer error. Makes this type of error easier to
disambiguate from a generic BadArgument.
