Chore: Simplify the error message. Make it more actionable

Author: izeigerman

sqlmesh/core/plan/definition.py

@@ -679,8 +679,7 @@ def _ensure_no_forward_only_revert(self) -> None:
                 )
             ):
                 raise PlanError(
-                    f"Detected an existing version of model '{name}' that has been previously superseded by a forward-only change. "
-                    "To proceed with the change, restamp this model's definition to produce a new version."
+                    f"Attempted to revert to an unrevertable version of model '{name}'. Run `sqlmesh plan` again to mitigate the issue."
                 )
 
     def _ensure_no_broken_references(self) -> None:

tests/core/test_plan.py

@@ -275,7 +275,7 @@ def test_forward_only_revert_not_allowed(make_snapshot, mocker: MockerFixture):
 
     with pytest.raises(
         PlanError,
-        match=r"Detected an existing version of model 'a' that has been previously superseded by a forward-only change.*",
+        match=r"Attempted to revert to an unrevertable version of model.*",
     ):
         Plan(context_diff_mock, forward_only=True)