Improve robustness of `is_channel_sendable()`

Residing in util/channels.py, this method is described by the docstring as serving the function of:

[Testing] for permission to send messages in a given channel.

Presently, its logic includes:

Channel object must be one of type TextChannel, VoiceChannel, Thread, or DMChannel.

Returns the channel on success, None for unknown, and otherwise False.

I'd like to improve this to:

Channel object must be one of: [TextChannel, VoiceChannel, Thread, DMChannel]

Returns the channel object on success, otherwise False, or raises TypeError.

Doing so would make it more robust as far as error and fallback handling is concerned.