fix(Webhook): throw an error if no token is available when it's required

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

Since Discord started sending application their own webhook's token again, #5731 is somewhat resolved.
This PR does the rest to resolve #5731.

• Use the optionally supplied token to Client#fetchWebhook if Discord did not include it in the payload. (i.e. fetching another application's webhook by id and token)
• Document when tokens are missing.
• Throw a more helpful error when trying to execute a message-related action on a webhook without a token.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)