bpo-45379: add custom error string for FROZEN_DISABLED (GH-29190)

Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Gareth Rees <gdr@garethrees.org>

Author: FFY00

Misc/NEWS.d/next/Core and Builtins/2021-10-23-13-49-00.bpo-45379.ZF7G3n.rst

@@ -0,0 +1,2 @@
+Clarify :exc:`ImportError` message when we try to explicitly import a
+frozen module but frozen modules are disabled.

Python/import.c

@@ -1153,9 +1153,11 @@ set_frozen_error(frozen_status status, PyObject *modname)
     switch (status) {
         case FROZEN_BAD_NAME:
         case FROZEN_NOT_FOUND:
-        case FROZEN_DISABLED:
             err = "No such frozen object named %R";
             break;
+        case FROZEN_DISABLED:
+            err = "Frozen modules are disabled and the frozen object named %R is not essential";
+            break;
         case FROZEN_EXCLUDED:
             err = "Excluded frozen object named %R";
             break;