Allow overridable checks to be CheckMessage (#497)

* Allow overridable checks to be CheckMessage

The checks framework expects return types to be "List[CheckMessage]". This
allows all overridable checks-returning methods to return the more general
"CheckMessage", instead of the "Error" subclass.

* Ignore an incorrect usage of the checks API in Django 2.2

django-stubs/contrib/admin/checks.pyi

@@ -1,7 +1,7 @@
 from typing import Any, List, Union, Iterable, Optional
 
 from django.contrib.admin.options import BaseModelAdmin
-from django.core.checks.messages import Error
+from django.core.checks.messages import CheckMessage, Error
 
 from django.apps.config import AppConfig
 
@@ -11,7 +11,7 @@ def check_admin_app(app_configs: Optional[Iterable[AppConfig]], **kwargs: Any) -
 def check_dependencies(**kwargs: Any) -> List[_CheckError]: ...
 
 class BaseModelAdminChecks:
-    def check(self, admin_obj: BaseModelAdmin, **kwargs: Any) -> List[_CheckError]: ...
+    def check(self, admin_obj: BaseModelAdmin, **kwargs: Any) -> List[CheckMessage]: ...
 
 class ModelAdminChecks(BaseModelAdminChecks): ...
 class InlineModelAdminChecks(BaseModelAdminChecks): ...

django-stubs/contrib/admin/options.pyi

@@ -11,7 +11,7 @@ from django.contrib.admin.sites import AdminSite
 from django.contrib.admin.views.main import ChangeList
 from django.contrib.auth.forms import AdminPasswordChangeForm
 from django.contrib.contenttypes.models import ContentType
-from django.core.checks.messages import Error
+from django.core.checks.messages import CheckMessage
 from django.core.paginator import Paginator
 from django.db.models.base import Model
 from django.db.models.fields.related import ForeignKey, ManyToManyField, RelatedField
@@ -75,7 +75,7 @@ class BaseModelAdmin:
     view_on_site: bool = ...
     show_full_result_count: bool = ...
     checks_class: Any = ...
-    def check(self, **kwargs: Any) -> List[Union[str, Error]]: ...
+    def check(self, **kwargs: Any) -> List[CheckMessage]: ...
     def formfield_for_dbfield(
         self, db_field: Field, request: Optional[HttpRequest], **kwargs: Any
     ) -> Optional[Field]: ...