[1.7.x] Fixed #8033 -- Explained app registry error during translation setup

Thanks Tim Graham and Aymeric Augustin for the review.
Backport of 9618d68b from master.

django/apps/registry.py

@@ -4,7 +4,7 @@ import sys
 import threading
 import warnings
 
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import AppRegistryNotReady, ImproperlyConfigured
 from django.utils import lru_cache
 from django.utils.deprecation import RemovedInDjango19Warning
 from django.utils._os import upath
@@ -116,7 +116,7 @@ class Apps(object):
         Raises an exception if the registry isn't ready.
         """
         if not self.ready:
-            raise RuntimeError("App registry isn't ready yet.")
+            raise AppRegistryNotReady()
 
     def get_app_configs(self):
         """

django/core/exceptions.py

@@ -12,6 +12,11 @@ class DjangoRuntimeWarning(RuntimeWarning):
     pass
 
 
+class AppRegistryNotReady(Exception):
+    """The django.apps registry is not populated yet"""
+    pass
+
+
 class ObjectDoesNotExist(Exception):
     """The requested object does not exist"""
     silent_variable_failure = True

django/utils/translation/trans_real.py

@@ -10,6 +10,7 @@ from threading import local
 import warnings
 
 from django.apps import apps
+from django.core.exceptions import AppRegistryNotReady
 from django.dispatch import receiver
 from django.test.signals import setting_changed
 from django.utils.deprecation import RemovedInDjango19Warning
@@ -181,7 +182,14 @@ def translation(language):
                     res.merge(t)
             return res
 
-        for app_config in reversed(list(apps.get_app_configs())):
+        try:
+            app_configs = reversed(list(apps.get_app_configs()))
+        except AppRegistryNotReady:
+            raise AppRegistryNotReady(
+                "The translation infrastructure cannot be initialized before the "
+                "apps registry is ready. Check that you don't make non-lazy "
+                "gettext calls at import time.")
+        for app_config in app_configs:
             apppath = os.path.join(app_config.path, 'locale')
             if os.path.isdir(apppath):
                 res = _merge(apppath)

docs/ref/applications.txt

@@ -382,9 +382,9 @@ Troubleshooting
 
 Here are some common problems that you may encounter during initialization:
 
-* ``RuntimeError: App registry isn't ready yet.`` This happens when importing
-  an application configuration or a models module triggers code that depends
-  on the app registry.
+* ``AppRegistryNotReady`` This happens when importing an application
+  configuration or a models module triggers code that depends on the app
+  registry.
 
   For example, :func:`~django.utils.translation.ugettext()` uses the app
   registry to look up translation catalogs in applications. To translate at

docs/releases/1.7.txt

@@ -934,7 +934,7 @@ script with::
     >>> import django
     >>> django.setup()
 
-Otherwise, you will hit ``RuntimeError: App registry isn't ready yet.``
+Otherwise, you will hit an ``AppRegistryNotReady`` exception.
 
 App registry consistency
 ^^^^^^^^^^^^^^^^^^^^^^^^