Update immutable tests.

stix2/test/test_indicator.py

@@ -129,7 +129,7 @@ def test_cannot_assign_to_indicator_attributes(indicator):
     with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         indicator.valid_from = dt.datetime.now()
 
-    assert str(excinfo.value) == "Cannot modify properties after creation."
+    assert str(excinfo.value) == "Cannot modify 'valid_from' property in <class 'stix2.sdo.Indicator'> after creation."
 
 
 def test_invalid_kwarg_to_indicator():

stix2/test/test_malware.py

@@ -92,7 +92,7 @@ def test_cannot_assign_to_malware_attributes(malware):
     with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         malware.name = "Cryptolocker II"
 
-    assert str(excinfo.value) == "Cannot modify properties after creation."
+    assert str(excinfo.value) == "Cannot modify 'name' property in <class 'stix2.sdo.Malware'> after creation."
 
 
 def test_invalid_kwarg_to_malware():

stix2/test/test_relationship.py

@@ -103,7 +103,7 @@ def test_cannot_assign_to_relationship_attributes(relationship):
     with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         relationship.relationship_type = "derived-from"
 
-    assert str(excinfo.value) == "Cannot modify properties after creation."
+    assert str(excinfo.value) == "Cannot modify 'relationship_type' property in <class 'stix2.sro.Relationship'> after creation."
 
 
 def test_invalid_kwarg_to_relationship():