plots: improve non-out error message

dvc/repo/plots/__init__.py

@@ -13,7 +13,8 @@
 class NotAPlotError(DvcException):
     def __init__(self, out):
         super().__init__(
-            f"'{out}' is not a plot. Use `dvc plots modify` to change that."
+            f"'{out}' is not a known plot. Use `dvc plots modify` to turn it "
+            "into one."
         )