It's a common practice to dismiss an error message and then want it back. We've probably all been guilty of clicking on "Don't show again" and then dismissing a message without really reading the whole thing, or reading it but not considering all of the ramifications. Fortunately, getting the message back is a simple process. Just go to Tools > Options to bring up the Options dialog box. Go to the System Options tab (it should be open by default), go to "Messages/Errors/Warnings", and check the appropriate box (or boxes) for the message(s) you'd like to bring back.
By the way, while I'm on the subject of these messages, while there are some that I think should have the "Don't ask again" option and don't, the "Confirm Delete" is one I don't think should have that option. Whether or not to delete child features and absorbed features when deleting a component from an Assembly, or whether or not to delete the sketch when deleting a Section View or Detail View from a Drawing, or if you want to delete a sketch along with a feature in a Part are just a few examples. I had that one deleted for a while, but never again.