fix(program-portals): prevent resume course links from 404ing (#27046)

* fix(program-portals): prevent resume course links from 404ing

From time to time course teams will delete a problem in an active
course. When this happens and that particular problem is the last
completed subsection for a learner, some naive attempts to provide a
"continue course" that takes the learner back to where they left off
may result in 404s. This checks whether a learner would be able to
access the block returned from completion data before we try to create
a link to that part of the course structure to hand to that learner.
It's used on the Master's program learner portal pages.

JIRA:EDUCATOR-5313

Co-authored-by: Zach Hancock <zhancock@edx.org>

common/djangoapps/student/helpers.py

@@ -15,9 +15,9 @@ from completion.utilities import get_key_to_last_completed_block
 from django.conf import settings
 from django.contrib.auth import load_backend
 from django.contrib.auth.models import User  # lint-amnesty, pylint: disable=imported-auth-user
-from django.core.exceptions import PermissionDenied, ObjectDoesNotExist
+from django.core.exceptions import ObjectDoesNotExist, PermissionDenied
 from django.core.validators import ValidationError
-from django.db import IntegrityError, transaction, ProgrammingError
+from django.db import IntegrityError, ProgrammingError, transaction
 from django.urls import NoReverseMatch, reverse
 from django.utils.translation import ugettext as _
 from pytz import UTC
@@ -44,11 +44,13 @@ from lms.djangoapps.certificates.api import (
     auto_certificate_generation_enabled,
 )
 from lms.djangoapps.certificates.data import CertificateStatuses
+from lms.djangoapps.course_blocks.api import get_course_blocks
 from lms.djangoapps.grades.api import CourseGradeFactory
 from lms.djangoapps.instructor import access
 from lms.djangoapps.verify_student.models import VerificationDeadline
 from lms.djangoapps.verify_student.services import IDVerificationService
 from lms.djangoapps.verify_student.utils import is_verification_expiring_soon, verification_for_datetime
+from openedx.core.djangoapps.content.block_structure.exceptions import UsageKeyNotInBlockStructure
 from openedx.core.djangoapps.site_configuration import helpers as configuration_helpers
 from openedx.core.djangoapps.theming.helpers import get_themes
 from openedx.core.djangoapps.user_authn.utils import is_safe_login_or_logout_redirect
@@ -766,10 +768,18 @@ def get_resume_urls_for_enrollments(user, enrollments):
     for enrollment in enrollments:
         try:
             block_key = get_key_to_last_completed_block(user, enrollment.course_id)
-            url_to_block = reverse(
-                'jump_to',
-                kwargs={'course_id': enrollment.course_id, 'location': block_key}
-            )
+            try:
+                block_data = get_course_blocks(user, block_key)
+            except UsageKeyNotInBlockStructure:
+                url_to_block = ''
+            else:
+                if block_key in block_data:
+                    url_to_block = reverse(
+                        'jump_to',
+                        kwargs={'course_id': enrollment.course_id, 'location': block_key}
+                    )
+                else:
+                    url_to_block = ''
         except UnavailableCompletionData:
             url_to_block = ''
         resume_course_urls[enrollment.course_id] = url_to_block