Stop JSONModel hiding attribute errors

`__getattr__` is called whenever an attribute error is raised.

This means that if something deep inside a property on a model raised
an attribute error, that error would be caught by `__getattr__`, which
would then raise an exception that looked like the property itself
didn’t exist. Very confusing.

The solution seems to be to override `__getattribute__` instead, which
handles _all_ attributes, not just those that aren’t explicitly defined.
We then only intervene if the desired attribute is one of the
`ALLOWED_PROPERTIES`, otherwise falling through to the built in methods
of the underlying `object`.

app/models/__init__.py

@@ -21,12 +21,27 @@ class JSONModel():
     def __eq__(self, other):
         return self.id == other.id
 
-    def __getattr__(self, attr):
-        if attr in self.ALLOWED_PROPERTIES:
-            return self._dict[attr]
-        raise AttributeError('`{}` is not a {} attribute'.format(
-            attr,
-            self.__class__.__name__.lower(),
+    def __getattribute__(self, attr):
+
+        try:
+            return super().__getattribute__(attr)
+        except AttributeError as e:
+            # Re-raise any `AttributeError`s that are not directly on
+            # this object because they indicate an underlying exception
+            # that we don’t want to swallow
+            if str(e) != "'{}' object has no attribute '{}'".format(
+                self.__class__.__name__, attr
+            ):
+                raise e
+
+        if attr in super().__getattribute__('ALLOWED_PROPERTIES'):
+            return super().__getattribute__('_dict')[attr]
+
+        raise AttributeError((
+            "'{}' object has no attribute '{}' and '{}' is not a field "
+            "in the underlying JSON"
+        ).format(
+            self.__class__.__name__, attr, attr
         ))
 
     def _get_by_id(self, things, id):