exceptions added for user input

exceptions.py

@@ -0,0 +1,22 @@
+class NotANumberException(Exception):
+    pass
+
+
+class SubjectNotExistingException(Exception):
+    pass
+
+
+class CategoryNotExistingException(Exception):
+    pass
+
+
+class WrongCaseException(Exception):
+    pass
+
+
+def convertible(string):
+    try:
+        float(string)
+        return True
+    except ValueError:
+        raise NotANumberException