metadata_manager: get rid of exceptions.py

They are nowhere being used now. The need for TooManyInstancesError can be fulfilled using the RuntimeError exception.

metadata_manager/exceptions.py

@@ -1,8 +0,0 @@
-class MetadataManagerException(Exception):
-    pass
-
-
-class TooManyInstancesError(MetadataManagerException):
-    def __init__(self, name: str):
-        message = f"{name} should be a singleton."
-        super().__init__(message)