makes Single.error() return a Single<empty>

[hearnden]

A Single produced by Single.error() should be assignable to any Single<T> type.

const willAlwaysError = Single.error(new Error());
const mySingle: Single<Foo> = willAlwaysError; // <-- this should always typecheck

This PR changes the default value of Single.error's type param to be empty, making it assignable to any Single<T> type.

This is the direct analogy of the default of never (same as empty) in TypeScript's Promise.reject:

reject<T = never>(reason?: any): Promise<T>;

[hearnden]

I also have a branch ready for a PR on DefinitelyTyped, if this PR LGTY: DefinitelyTyped/DefinitelyTyped#42584

[stevegury]

Thanks @hearnden for the improvement!
Based on my limited knowledge in Flow, this looks good to me.