add InvalidArgumentException, fixes #15

sloth/core/exceptions.py

@@ -9,3 +9,7 @@ class ImproperlyConfigured(Exception):
 class NotImplementedException(Exception):
     """This function/method/class has not been implemented yet."""
     pass
+
+class InvalidArgumentException(Exception):
+    """The argument is invalid."""
+    pass

sloth/gui/annotationscene.py

@@ -3,6 +3,7 @@ from PyQt4.QtGui import *
 from PyQt4.QtCore import *
 from sloth.items import *
 from sloth.annotations.model import TypeRole, ImageRole
+from sloth.core.exceptions import InvalidArgumentException
 import math
 import okapy
 from okapy.guiqt.utilities import toQImage