sql/schemachange: skip constraint validation if column generation fails

Previously, we would still try to validate constraints, if any of
the generated columns could be evaluated. To address this, this
patch will skip constraint validation in these scenarios.
<what you did about it: To address this, this patch ...>

Release note: None

pkg/workload/schemachange/error_screening.go

@@ -326,11 +326,8 @@ GROUP BY name;
 	for range constraints {
 		constraintTuples = append(constraintTuples, make(map[string]struct{}))
 	}
-
 	for _, row := range rows {
-		if err != nil {
-			return false, nil, err
-		}
+		hasGenerationError := false
 		// Put values to be inserted into a column name to value map to simplify lookups.
 		columnsToValues := map[string]string{}
 		for i := 0; i < len(columns); i++ {
@@ -343,9 +340,12 @@ GROUP BY name;
 				continue
 			}
 			evalTxn, err := tx.Begin(ctx)
+			if err != nil {
+				return false, nil, err
+			}
 			newCols[colInfo.name], err = og.generateColumn(ctx, tx, colInfo, columnsToValues)
-			evalTxn.Rollback(ctx)
 			if err != nil {
+				_ = evalTxn.Rollback(ctx)
 				var pgErr *pgconn.PgError
 				if !errors.As(err, &pgErr) {
 					return false, nil, err
@@ -359,15 +359,21 @@ GROUP BY name;
 						{code: pgcode.MakeCode(pgErr.Code), condition: true},
 					}...,
 				)
+				hasGenerationError = true
 				continue
 			}
+			err = evalTxn.Commit(ctx)
 			if err != nil {
 				return false, nil, err
 			}
 		}
 		for k, v := range newCols {
 			columnsToValues[k] = v
 		}
+		// Skip over constraint validation, since we know an expression is bad here.
+		if hasGenerationError {
+			continue
+		}
 		// Next validate the uniqueness of both constraints and index expressions.
 		for constraintIdx, constraint := range constraints {
 			nonTupleConstraint := constraint[0]