sql: fix NewUniquenessConstraintViolationError reporting

Release note (bug fix):
If NewUniquenessConstraintViolationError cannot initialize a row fetcher
it will only report this error to the client without wrappibng it with
information about the actual constraint violation which can be confusing.

Old error:
`ERROR: column-id "2" does not exist`

New Error:
```
ERROR: duplicate key value (b)=(couldn't fetch value: column-id "2"
does not exist) violates unique constraint "t_secondary"
```

See cockroachdb#46276.

pkg/sql/logictest/testdata/logic_test/drop_index

@@ -323,3 +323,19 @@ CREATE TABLE drop_index_test(a int); CREATE INDEX drop_index_test_index ON drop_
 ----
 NOTICE: the data for dropped indexes is reclaimed asynchronously
 HINT: The reclamation delay can be customized in the zone configuration for the table.
+
+# test correct error reporting from NewUniquenessConstraintViolationError; see #46376
+subtest new_uniqueness_constraint_error
+
+statement ok
+CREATE TABLE t (a INT PRIMARY KEY, b DECIMAL(10,1) NOT NULL DEFAULT(0), UNIQUE INDEX t_secondary (b), FAMILY (a, b));
+INSERT INTO t VALUES (100, 500.5);
+
+statement ok
+BEGIN;
+DROP INDEX t_secondary CASCADE;
+ALTER TABLE t DROP COLUMN b;
+INSERT INTO t SELECT a + 1 FROM t;
+
+statement error pq: duplicate key value
+UPSERT INTO t SELECT a + 1 FROM t;

pkg/sql/row/errors.go

@@ -123,7 +123,11 @@ func NewUniquenessConstraintViolationError(
 		&sqlbase.DatumAlloc{},
 		tableArgs,
 	); err != nil {
-		return err
+		return pgerror.Newf(pgcode.UniqueViolation,
+			"duplicate key value (%s)=(%v) violates unique constraint %q",
+			strings.Join(index.ColumnNames, ","),
+			errors.Wrapf(err, "couldn't fetch value"),
+			index.Name)
 	}
 	f := singleKVFetcher{kvs: [1]roachpb.KeyValue{{Key: key}}}
 	if value != nil {