Expanded QuerySet.explain() error message if a backend supports no formats.

django/db/backends/base/operations.py

@@ -754,6 +754,10 @@ class BaseDatabaseOperations:
                 msg = "%s is not a recognized format." % normalized_format
                 if supported_formats:
                     msg += " Allowed formats: %s" % ", ".join(sorted(supported_formats))
+                else:
+                    msg += (
+                        f" {self.connection.display_name} does not support any formats."
+                    )
                 raise ValueError(msg)
         if options:
             raise ValueError("Unknown options: %s" % ", ".join(sorted(options.keys())))

tests/queries/test_explain.py

@@ -74,6 +74,8 @@ class ExplainTests(TestCase):
             msg += " Allowed formats: %s" % ", ".join(
                 sorted(connection.features.supported_explain_formats)
             )
+        else:
+            msg += f" {connection.display_name} does not support any formats."
         with self.assertRaisesMessage(ValueError, msg):
             Tag.objects.explain(format="does not exist")