Fix a bug where AR::RecordNotSaved loses error messages

Since 3e30c5d, it started ignoring the given error message. This commit
changes the behavior of AR::RecordNotSaved#initialize so that it no
longer loses the given error message.
Merge pull request #17808 from yuki24/fix-bug-where-record-not-saved-loses-error-message

Fixed a bug where AR::RecordNotSaved loses the given error message