Improve error message for invalid reference types

stix2/properties.py

@@ -590,18 +590,26 @@ class ReferenceProperty(Property):
                 ) and obj_type not in specifics
             ) or obj_type in blacklist_exceptions
 
-        if not type_ok:
-            raise ValueError(
-                "The type-specifying prefix '%s' for this property is not "
-                "valid" % obj_type,
-            )
-
         # We need to figure out whether the referenced object is custom or
         # not.  No good way to do that at present... just check if
         # unregistered and for the "x-" type prefix, for now?
         has_custom = not is_object(obj_type, self.spec_version) \
             or obj_type.startswith("x-")
 
+        if not type_ok:
+            types = self.specifics.union(self.generics)
+            types = ", ".join(x.name if isinstance(x, STIXTypeClass) else x for x in types)
+            if self.auth_type == self._WHITELIST:
+                msg = "not one of the valid types for this property: %s." % types
+            else:
+                msg = "one of the invalid types for this property: %s." % types
+            if not allow_custom and has_custom:
+                msg += " A custom object type may be allowed with allow_custom=True."
+            raise ValueError(
+                "The type-specifying prefix '%s' for this property is %s"
+                % (obj_type, msg),
+            )
+
         if not allow_custom and has_custom:
             raise CustomContentError(
                 "reference to custom object type: " + obj_type,

stix2/test/v21/test_observed_data.py

@@ -208,7 +208,7 @@ def test_observed_data_example_with_bad_refs():
 
     assert excinfo.value.cls == stix2.v21.Directory
     assert excinfo.value.prop_name == "contains_refs"
-    assert "The type-specifying prefix 'monkey' for this property is not valid" in excinfo.value.reason
+    assert "The type-specifying prefix 'monkey' for this property is not" in excinfo.value.reason
 
 
 def test_observed_data_example_with_non_dictionary():