fix(Util): throw token invalid for fetching rec. shard amount

[almostSouji]

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

• ⚠fixes Using the ShardManager, if the token is invalid, it will return an error that doesnt describe at all what is happening. #4778

As #4778 has brough to light we currently throw the response if an invalid token is provided when fetching the recommended amount of shards from the API.

While changing the kind of error to throw here could be framed as major i am so bold to say that this can not be deliberate design (as it results in the rather unhelpful #<Response> being thrown) and thus will tag it as patch and consider it a fix rather than a feature.

discord.js/src/util/Util.js

Lines 246 to 247 in b0ab37d

	if (res.ok) return res.json();
	throw res;

Seeing that providing an invalid token will result in a 401 response adding a line to throw TOKEN_INVALID instead if that's the case provides more context to the user but still enables other faults to be propagated.

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.