fix(typings): tweak typings to work with ts@next

See microsoft/TypeScript#23592

Author: ethanresnick

src/util/errors.ts

@@ -1,9 +1,6 @@
 import APIError, { Opts } from '../types/APIError';
 
-export type Diff<T extends string, U extends string> =
-  ({ [P in T]: P } & { [P in U]: never } & { [x: string]: never })[T];
-
-export type Omit<T, K extends keyof T> = Pick<T, Diff<keyof T, K>>;
+export type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
 export type ErrorOpts = Omit<Opts, "title" | "status" | "typeUri" | "code">;
 
 /*