get_model: Raise LookupError instead of ImportError

oscar.core.loading.get_model is a thin wrapper around Django 1.6's
get_model function. The latter returns None if the model can't be found,
whereas the Oscar variant loudly failed with an ImportError.
With Django's app refactor, most of that behaviour in Django has been
changed, and Oscar's get_model falls back to Django's
get_registered_model. But the latter raises a LookupError.

To avoid having to catch both errors, Oscar's get_model now raises a
LookupError no matter what Django version is used. This should have
minimal impact on backwards compatibility, as get_model isn't really
used outside of Oscar core.

docs/source/releases/v0.8.rst

@@ -455,6 +455,10 @@ Misc
   issue, as the model is only used while an range upload is in progress. If
   you need to keep the data, ensure you migrate it across.
 
+* ``oscar.core.loading.get_model`` now raises a ``LookupError`` instead of an
+  ``ImportError`` if a model can't be found. That brings it more in line with
+  what Django does since the app refactor.
+
 .. _rewritten: https://github.com/tangentlabs/django-oscar/commit/d8b4dbfed17be90846ea4bc47b5f7b39ad944c24
 
 Basket line stockrecords

oscar/core/loading.py

@@ -260,10 +260,11 @@ def feature_hidden(feature_name):
 
 
 # The following section is concerned with offering both the
-# get_model(app_label, model_name) and is_model_registered(app_label, model_name)
-# methods. Because the Django internals dramatically changed in the Django 1.7
-# app refactor, we distinguish based on the Django version and declare
-# a total of four methods that hopefully do mostly the same
+# get_model(app_label, model_name) and
+# is_model_registered(app_label, model_name) methods. Because the Django
+# internals dramatically changed in the Django 1.7 app refactor, we distinguish
+# based on the Django version and declare a total of four methods that
+# hopefully do mostly the same
 
 
 if django.VERSION < (1, 7):
@@ -275,10 +276,11 @@ if django.VERSION < (1, 7):
         Gets a model class by it's app label and model name. Fails loudly if
         the model class can't be imported.
         This is merely a thin wrapper around Django's get_model function.
+        Raises LookupError if model isn't found.
         """
         model = django_get_model(app_label, model_name, *args, **kwargs)
         if model is None:
-            raise ImportError(
+            raise LookupError(
                 "{app_label}.{model_name} could not be imported.".format(
                     app_label=app_label, model_name=model_name))
         return model
@@ -302,6 +304,7 @@ else:
         which makes it safe to call when the registry is being populated.
         All other methods to access models might raise an exception about the
         registry not being ready yet.
+        Raises LookupError if model isn't found.
         """
         return apps.get_registered_model(app_label, model_name)