Docs - add explanation of difference between FAIL and ERROR cases

It has come to my attention that we do not clearly define the difference between a failure and an error in the results. Need to add explanation about what each case is and how to best debug.

Start by locating the documentation that describes test results and the FAIL and ERROR cases. Add definitions for both outcomes and guidance on debugging them; the work is done when readers can distinguish the cases and know how to investigate each one.

Written by the indexing model from the issue text.