eval: Fix crash on missing printValue tBlackhole case

Fixes #8119

src/libexpr/eval.cc

@@ -173,7 +173,17 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
     case tFloat:
         str << fpoint;
         break;
+    case tBlackhole:
+        // Although we know for sure that it's going to be an infinite recursion
+        // when this value is accessed _in the current context_, it's likely
+        // that the user will misinterpret a simpler «infinite recursion» output
+        // as a definitive statement about the value, while in fact it may be
+        // a valid value after `builtins.trace` and perhaps some other steps
+        // have completed.
+        str << "«potential infinite recursion»";
+        break;
     default:
+        printError("Nix evaluator internal error: Value::print(): invalid value type %1%", internalType);
         abort();
     }
 }