Improve mock usability in tests

[feldentm-SAP]

Add stacktrace on fatal exit of mocked call to allow maintainers of tests using this API to understand what is actually going wrong

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[feldentm-SAP]

@codyoss please have a look at it; not having it in an official release is burning a lot of my time because I have to rework tests from a different team and just getting the info that something has not happened the right way is not enough in such a scenario