Review and improve testwhat messaging

[filipsch]

Still TBD what is bad and what needs to improve, but it's 100% sure that something can be better.

I checked the following out briefly (on the suggestion of @richierocks), but I'm not sure if they can help in human-understandable messages that explain what is wrong with their code:

• daff (https://cran.r-project.org/web/packages/daff)
• diffObj (https://cran.r-project.org/web/packages/diffobj)
• datacompareR (https://github.com/capitalone/dataCompareR)