fix(GuildEmojiManager): check for guild in methods that use it

[vaporoxx]

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

Client#emojis and Guild#emojis both return a GuildEmojiManager, however the current code base does not account for former not having a valid guild property. client.emojis.create(...) before resulted in DiscordAPIError: 405: Method Not Allowed, this PR aims to fix that by checking if a guild is present before using it for methods and throwing a custom error.

Consideration

There's also the option to create two separate classes for Client#emojis and Guild#emojis, I've decided to take this approach for now but I'm open to suggestions.

Status

• Code changes have been tested against the Discord API
• I know how to update typings and have done so

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.