Avoid breaking with bad custom constructors

[fregante]

• Blocked by and based on Stops making non-Error properties non-enumerable #102
• Also fixes Wrong typescript declaration for errorConstructors #78

Breaking change.

As the readme suggested, adding bad constructors to this map would break the entire application. Now it fails exactly when they're added.

Before

import { errorConstructors } from "serialize-error"

errorConstructors.set("MessengerError", MessengerError);

After

import { addKnownErrorConstructor } from "serialize-error";

addKnownErrorConstructor(MessengerError);