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

[CLAassistant]

All committers have signed the CLA.

[feldentm-SAP]

migrate PR from golang/mock#680

[feldentm-SAP]

@JacobOaks I consider this change a bug fix and would be allowed to donate it to the project without signing the CLA. Signing an Uber CLA is currently not that easy.

[JacobOaks]

Hi @feldentm-SAP - looks like Github is intent on closing this PR on my behalf for some reason (I did not close this and when I try to re-open it immediately closes it). Let me look into it

[dobrinkamladenova]

@JacobOaks, to whom may I forward the executed copy of the CLA?

[feldentm-SAP]

I managed to progress on the legal side, but are now facing technical issues with signing the CLA.

[feldentm-SAP]

@JacobOaks do you know why I'm not allowed to reopen this PR? do you have an idea why I'm presented an empty CLA UI with no means to sign anything?

[feldentm-SAP]

@JacobOaks I finally managed to sign the CLA and would love this PR to get merged

[feldentm-SAP]

@JacobOaks thanks for trying, but I created an identical PR in the meantime to work around this issue: #145