Use consistent errors for observable extensions

Whether or not the Observable type is in the EXT_MAP already, using a
custom extension without also using allow_custom=True should result in
the same behavior/error message.

stix2/test/test_custom.py

@@ -826,9 +826,10 @@ def test_parse_observable_with_custom_extension():
     assert parsed.extensions['x-new-ext'].property2 == 12
 
 
-def test_parse_observable_with_unregistered_custom_extension():
-    input_str = """{
-        "type": "domain-name",
+@pytest.mark.parametrize("data", [
+    # URL is not in EXT_MAP
+    """{
+        "type": "url",
         "value": "example.com",
         "extensions": {
             "x-foobar-ext": {
@@ -836,13 +837,25 @@ def test_parse_observable_with_unregistered_custom_extension():
                 "property2": 12
             }
         }
-    }"""
-
+    }""",
+    # File is in EXT_MAP
+    """{
+        "type": "file",
+        "name": "foo.txt",
+        "extensions": {
+            "x-foobar-ext": {
+                "property1": "foo",
+                "property2": 12
+            }
+        }
+    }""",
+])
+def test_parse_observable_with_unregistered_custom_extension(data):
     with pytest.raises(ValueError) as excinfo:
-        stix2.parse_observable(input_str)
+        stix2.parse_observable(data)
     assert "Can't parse unknown extension type" in str(excinfo.value)
 
-    parsed_ob = stix2.parse_observable(input_str, allow_custom=True)
+    parsed_ob = stix2.parse_observable(data, allow_custom=True)
     assert parsed_ob['extensions']['x-foobar-ext']['property1'] == 'foo'
     assert not isinstance(parsed_ob['extensions']['x-foobar-ext'], stix2.core._STIXBase)
 

stix2/test/test_properties.py

@@ -412,7 +412,7 @@ def test_extension_property_invalid_type():
                 'pe_type': 'exe'
             }}
         )
-    assert 'no extensions defined' in str(excinfo.value)
+    assert "Can't parse unknown extension" in str(excinfo.value)
 
 
 def test_extension_at_least_one_property_constraint():

stix2/v20/observables.py

@@ -73,8 +73,7 @@ class ExtensionsProperty(DictionaryProperty):
         if dictified == {}:
             raise ValueError("The extensions property must contain a non-empty dictionary")
 
-        if self.enclosing_type in EXT_MAP:
-            specific_type_map = EXT_MAP[self.enclosing_type]
+        specific_type_map = EXT_MAP.get(self.enclosing_type, {})
         for key, subvalue in dictified.items():
             if key in specific_type_map:
                 cls = specific_type_map[key]
@@ -91,8 +90,6 @@ class ExtensionsProperty(DictionaryProperty):
                     raise ValueError("Cannot determine extension type.")
             else:
                 raise CustomContentError("Can't parse unknown extension type: {}".format(key))
-        else:
-            raise ValueError("The enclosing type '%s' has no extensions defined" % self.enclosing_type)
         return dictified