Custom errors should inherit js `Error`

[Wiktor Obrębski]

In some cases, especially when using .unsafePromise(), the custom domain errors can be thrown.
The function should not be used in normal application flow, but can be useful in tests.

https://mochajs.org/ test runner do not works well with throwed / rejected errors that do not inherit by Error.
This is good enough reason to change proposed error classes style in the library documentation.
Proposition of new error style:

class InvalidCredentialsError extends Error {
  name = 'InvalidCredentials' as const;
}