refactor: remove error_descriptor_class and NonStaffErrorBlock

It's safe to remove this because non-staff [1] users cannot access [2]
an `ErrorBlock`. We were able to reproduce this with and without this commit
with the following results:
1. Staff users were seeing the `ErrorBlock`.
2. Non-staff users were getting an empty `<div class="vert-mod"></div>`.

In theory, error blocks should be hidden in the Learning MFE because of this
option [3]. However, when we manually set `hide_access_error_blocks` to
`False`, we kept getting identical results (with and without this commit), so
it looks that the removal `NonStaffErrorBlock` was just omitted at some point.

[1] a4ec4c1b8e/lms/djangoapps/courseware/access.py (L419-L436)
[2] a4ec4c1b8e/lms/djangoapps/courseware/access.py (L150-L151)
[3] 92ca176fde/lms/djangoapps/courseware/views/views.py (L1547-L1551)

cms/djangoapps/contentstore/views/preview.py

@@ -17,7 +17,6 @@ from xblock.exceptions import NoSuchHandlerError
 from xblock.runtime import KvsFieldData
 
 from xmodule.contentstore.django import contentstore
-from xmodule.error_module import ErrorBlock
 from xmodule.exceptions import NotFoundError, ProcessingError
 from xmodule.modulestore.django import ModuleI18nService, modulestore
 from xmodule.partitions.partitions_service import PartitionService
@@ -218,7 +217,6 @@ def _preview_module_system(request, descriptor, field_data):
         # Set up functions to modify the fragment produced by student_view
         wrappers=wrappers,
         wrappers_asides=wrappers_asides,
-        error_descriptor_class=ErrorBlock,
         # Get the raw DescriptorSystem, not the CombinedSystem
         descriptor_runtime=descriptor._runtime,  # pylint: disable=protected-access
         services={