Fixed #27661 -- Moved FileSystemFinder's ImproperlyConfigured exceptions to system checks.

Thanks Simon Charette, Mariusz Felisiak, Tim Graham, and Adam Johnson
for review.

tests/staticfiles_tests/test_checks.py

@@ -0,0 +1,77 @@
+from unittest import mock
+
+from django.conf import settings
+from django.contrib.staticfiles.checks import check_finders
+from django.contrib.staticfiles.finders import BaseFinder
+from django.core.checks import Error
+from django.test import SimpleTestCase, override_settings
+
+
+class FindersCheckTests(SimpleTestCase):
+
+    def test_base_finder_check_not_implemented(self):
+        finder = BaseFinder()
+        msg = 'subclasses may provide a check() method to verify the finder is configured correctly.'
+        with self.assertRaisesMessage(NotImplementedError, msg):
+            finder.check()
+
+    def test_check_finders(self):
+        """check_finders() concatenates all errors."""
+        error1 = Error('1')
+        error2 = Error('2')
+        error3 = Error('3')
+
+        def get_finders():
+            class Finder1(BaseFinder):
+                def check(self, **kwargs):
+                    return [error1]
+
+            class Finder2(BaseFinder):
+                def check(self, **kwargs):
+                    return []
+
+            class Finder3(BaseFinder):
+                def check(self, **kwargs):
+                    return [error2, error3]
+
+            class Finder4(BaseFinder):
+                pass
+
+            return [Finder1(), Finder2(), Finder3(), Finder4()]
+
+        with mock.patch('django.contrib.staticfiles.checks.get_finders', get_finders):
+            errors = check_finders(None)
+            self.assertEqual(errors, [error1, error2, error3])
+
+    def test_no_errors_with_test_settings(self):
+        self.assertEqual(check_finders(None), [])
+
+    @override_settings(STATICFILES_DIRS='a string')
+    def test_dirs_not_tuple_or_list(self):
+        self.assertEqual(check_finders(None), [
+            Error(
+                'The STATICFILES_DIRS setting is not a tuple or list.',
+                hint='Perhaps you forgot a trailing comma?',
+                id='staticfiles.E001',
+            )
+        ])
+
+    @override_settings(STATICFILES_DIRS=['/fake/path', settings.STATIC_ROOT])
+    def test_dirs_contains_static_root(self):
+        self.assertEqual(check_finders(None), [
+            Error(
+                'The STATICFILES_DIRS setting should not contain the '
+                'STATIC_ROOT setting.',
+                id='staticfiles.E002',
+            )
+        ])
+
+    @override_settings(STATICFILES_DIRS=[('prefix', settings.STATIC_ROOT)])
+    def test_dirs_contains_static_root_in_tuple(self):
+        self.assertEqual(check_finders(None), [
+            Error(
+                'The STATICFILES_DIRS setting should not contain the '
+                'STATIC_ROOT setting.',
+                id='staticfiles.E002',
+            )
+        ])