Resurrect the checks from xls/dslx/diagnostics:maybe_explain_error

This extra error message logic is effectively dead code with TIv2 turned on. Taking it out of the loop was not intentional; we didn't realize it existed when building TIv2. We are going to remove it for the time being because it's nontrivial to wean the test off of its use of TIv1 internals. We should resurrect it or move the functionality somewhere else.