Add custom exceptions to the document state app.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_states/exceptions.py

@@ -0,0 +1,9 @@
+from __future__ import unicode_literals
+
+
+class WorkflowException(Exception):
+    """Base exception for the document states app"""
+
+
+class WorkflowStateActionError(WorkflowException):
+    """Raise for errors during exection of workflow state actions"""