Better error message when Django is not configured. Closes #277

if Django isn't configured, that is DJANGO_SETTINGS_MODULE isn't
available in the environment, then raise a RuntimeError with message

DJANGO_SETTINGS_MODULE required for resolving ForeignKey string
references, see Usage section in README!

pylint_django/transforms/foreignkey.py

@@ -6,6 +6,7 @@
 )
 from astroid.nodes import ClassDef, Attribute
 
+from django.core.exceptions import ImproperlyConfigured
 from pylint_django.utils import node_is_subclass
 
 
@@ -106,6 +107,9 @@ def infer_key_classes(node, context=None):
                     # 'auth.models', 'User' which works nicely with the `endswith()`
                     # comparison below
                     module_name += '.models'
+                except ImproperlyConfigured:
+                    raise RuntimeError("DJANGO_SETTINGS_MODULE required for resolving ForeignKey "
+                                       "string references, see Usage section in README!")
 
                 # ensure that module is loaded in astroid_cache, for cases when models is a package
                 if module_name not in MANAGER.astroid_cache: