Added Exception for not ready methods

pyrmv/errors/not_ready.py

@@ -0,0 +1,9 @@
+class NotReadyYetError(Exception):
+    """
+    This method is not finished yet.
+    """
+    def __init__(self):
+        super().__init__(self.__doc__)
+
+    def __str__(self):
+        return self.__doc__