Make Immutable error more descriptive. Also fixes #13

stix2/base.py

@@ -128,7 +128,7 @@ class _STIXBase(collections.Mapping):
 
     def __setattr__(self, name, value):
         if name != '_inner' and not name.startswith("_STIXBase__"):
-            raise ImmutableError
+            raise ImmutableError(self.__class__, name)
         super(_STIXBase, self).__setattr__(name, value)
 
     def __str__(self):

stix2/exceptions.py

@@ -47,8 +47,14 @@ class ExtraPropertiesError(STIXError, TypeError):
 class ImmutableError(STIXError, ValueError):
     """Attempted to modify an object after creation"""
 
-    def __init__(self):
-        super(ImmutableError, self).__init__("Cannot modify properties after creation.")
+    def __init__(self, cls, key):
+        super(ImmutableError, self).__init__()
+        self.cls = cls
+        self.key = key
+
+    def __str__(self):
+        msg = "Cannot modify '{0.key}' property in {0.cls} after creation."
+        return msg.format(self)
 
 
 class DictionaryKeyError(STIXError, ValueError):