plots: improve non-out error message (#4115)

* plot: link to cmd ref in output

* plots: change non-plot output error msg
per https://github.com/iterative/dvc/pull/3994#discussion_r438917424

* Update dvc/repo/plots/__init__.py

Co-authored-by: Ruslan Kuprieiev <[email protected]>

Co-authored-by: Ruslan Kuprieiev <[email protected]>

dvc/repo/plots/__init__.py

@@ -13,7 +13,8 @@ logger = logging.getLogger(__name__)
 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."
         )