Add support for passing a function to stub.throws(...)

[noisecapella]

Purpose (TL;DR) - mandatory

Fixes #1507
This PR adds functionality to allow stub.throws(...) to accept a function as an argument. This function is invoked at the point that the exception is thrown which allows the end user to have a more descriptive stack trace for the error. This also delays instantiating the Error until it is thrown (except when the user has already instantiated it) to create a more detailed stack trace.

Background (Problem in detail) - optional

At least in node, the stack trace for an Error is created when the object is instantiated rather than when it's thrown (from here). The stack trace is sometimes useful in figuring out what pieces of code are involved with an error. Currently errors used in throws(...) are instantiated by the caller or soon after in the setup of the stub which results in a stack trace showing where the setup occurred rather than where the problem occurred.

Solution - optional

In cases where the exception is not passed into throws(...), a function which will instantiate the exception is used internally instead and assigned to a new field exceptionCreator. When the exception is thrown exceptionCreator is instantiated and the result is assigned to exception to allow inspection via toString or other means.

How to verify - mandatory

To see the difference in stack trace see the test I included in the linked issue.

[fatso83]

Looks good, but need to test it to get a feel for it :-)

[mantoni]

Nice work! I'm missing test cases verifying that the error object is now created lazily for all three cases.

[noisecapella]

I added some tests, are those assertions what you were thinking of?

[mantoni]

In general, yes. However, you're now testing the implementation details, not the behavior. I was thinking of asserting the point in time a given function is actually called, or stubbing the Error constructor and checking the point in time when it's created.

[noisecapella]

I see, I'll make the change

[fatso83]

Think those tests look much better.

[mantoni]

Great tests! Thank you very much 👍

[fatso83]

This introduced regression #1526. Mind taking a look at it, @noisecapella?

[noisecapella]

Yes I'll take a look