With the release of V9 we've seen new items in the solution structure and package. One such change is the Messages under each entity in the solution.

Messages give the ability to change the content displayed when the user sees these out of the box messages. It includes user interface text and error messages. It gives you the default display string and allows you to enter a custom display string and then a comment to explain the change. Editing is very straight forward and looks like this:

It's worth noting that entity messages aren't available for custom entities and that your only able to edit existing messages.