Fix duplicate domain error handling

Ensures a more applicable error message when creating a new domain
that exists already (in another project)

Change-Id: Id8622d17761b01bf9e1ddecac83b4c068da63288
Closes-Bug: 1459786

designate/exceptions.py

@@ -197,6 +197,14 @@ class Forbidden(Base):
     expected = True
 
 
+class IllegalChildDomain(Forbidden):
+    error_type = 'illegal_child'
+
+
+class IllegalParentDomain(Forbidden):
+    error_type = 'illegal_parent'
+
+
 class IncorrectZoneTransferKey(Forbidden):
     error_type = 'invalid_key'