database/sql: improve unimplemented LastInsertId error message

The old one was rather confusing - it makes it sound like the user has done something wrong.

Change-Id: Ibc7411f4f1d5f6c66fbcaac64bb05b0743354418
GitHub-Last-Rev: 09290accddb23848ee80d641e4f2bcf6ef686e01
GitHub-Pull-Request: golang/go#27979
Reviewed-on: https://go-review.googlesource.com/c/139102Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/driver/driver.go

@@ -469,7 +469,7 @@ type RowsAffected int64
 var _ Result = RowsAffected(0)
 
 func (RowsAffected) LastInsertId() (int64, error) {
-	return 0, errors.New("no LastInsertId available")
+	return 0, errors.New("LastInsertId is not supported by this driver")
 }
 
 func (v RowsAffected) RowsAffected() (int64, error) {