Specific response assertion error messages

[PietervdWerk]

I've been getting the error "failed to get response body: type assertion error" and it took me quite a while to figure out what the problem was. I've copied my policy into a playground and it worked fine there (in hindsight that's obvious).

I propose a more verbose and specific error message when assertion fails. I've taken the liberty to come with some suggestions.

[PietervdWerk]

I've updated the error messages to be "expected x, got y". Let me me know what you think :).

[ashutosh-narkar]

@PietervdWerk this would be a good change to get in. Are you able to update the tests?

[PietervdWerk]

Yes tomorrow! (I had a few days off, back to work tomorrow.)