diff --git a/dvc/exceptions.py b/dvc/exceptions.py
index 7568ebc8e..d92b23761 100644
--- a/dvc/exceptions.py
+++ b/dvc/exceptions.py
@@ -7,6 +7,7 @@ class DvcException(Exception):
 
     def __init__(self, msg, *args):
         assert msg
+        self.msg = msg
         super().__init__(msg, *args)
 
 
diff --git a/dvc/stage/exceptions.py b/dvc/stage/exceptions.py
index 1f180b2c6..c842a08d3 100644
--- a/dvc/stage/exceptions.py
+++ b/dvc/stage/exceptions.py
@@ -80,9 +80,7 @@ class MissingDataSource(DvcException):
         super().__init__(msg)
 
 
-class StageNotFound(KeyError, DvcException):
-    __str__ = DvcException.__str__
-
+class StageNotFound(DvcException, KeyError):
     def __init__(self, file, name):
         self.file = file.relpath
         self.name = name
@@ -90,6 +88,11 @@ class StageNotFound(KeyError, DvcException):
             f"Stage '{self.name}' not found inside '{self.file}' file"
         )
 
+    def __str__(self):
+        # `KeyError` quotes the message
+        # see: https://bugs.python.org/issue2651
+        return self.msg
+
 
 class StageNameUnspecified(DvcException):
     def __init__(self, file):