rxjs-throw-error should not complain about throwing subtypes of Error

[felixfbecker]

throw Object.assign(new Error('Not Found'), { code: 'NOT_FOUND' })

currently creates a complaint, but shouldn't because the type of this exception is Error & { code: string }, which is a subtype of Error.

[cartant]

Note for self: the bug is most likely in the implementation of couldBeType, as Object.assign has this definition:

assign<T, U>(target: T, source: U): T & U;

[cartant]

This should be fixed in 4.17.1.