Override __str__ implementation for StageNotFound (iterative#5289)

This happens because KeyError quotes the error message.
We'd still like to keep StageNotFound being a KeyError
semantics, but workaround the __str__ implementation.

Mypy was having some problem with the previous `__str__`
override.

dvc/exceptions.py

@@ -7,6 +7,7 @@ class DvcException(Exception):
 
     def __init__(self, msg, *args):
         assert msg
+        self.msg = msg
         super().__init__(msg, *args)
 
 

dvc/stage/exceptions.py

@@ -80,16 +80,19 @@ def __init__(self, missing_files):
         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
         super().__init__(
             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):