refactor: cleanup & general refactor

[NotSugden]

reference: this pr is apart of the chain of prs requested from https://github.com/discordjs/RPC/pull/89#issuecomment-693537755

This PR is a general refactor to cleanup some code, return correct values as outlined by the documentation (see previous PR #90)

This PR also adds (back?) the custom Error class to handle errors

important note: some of the documentation around the diff changes will be incorrect, #90 has these changes and should be merged before this

[NotSugden]

i suck at git - apologies for that