feat: show error on courses with unknown proctoring providers (#35445)

cms/djangoapps/models/settings/course_metadata.py

@@ -490,6 +490,24 @@ class CourseMetadata:
             enable_proctoring = block.enable_proctored_exams
 
         if enable_proctoring:
+
+            if proctoring_provider_model:
+                proctoring_provider = proctoring_provider_model.get('value')
+            else:
+                proctoring_provider = block.proctoring_provider
+
+            # If the proctoring provider stored in the course block no longer
+            # matches the available providers for this instance, show an error
+            if proctoring_provider not in available_providers:
+                message = (
+                    f'The proctoring provider configured for this course, \'{proctoring_provider}\', is not valid.'
+                )
+                errors.append({
+                    'key': 'proctoring_provider',
+                    'message': message,
+                    'model': proctoring_provider_model
+                })
+
             # Require a valid escalation email if Proctortrack is chosen as the proctoring provider
             escalation_email_model = settings_dict.get('proctoring_escalation_email')
             if escalation_email_model:
@@ -497,11 +515,6 @@ class CourseMetadata:
             else:
                 escalation_email = block.proctoring_escalation_email
 
-            if proctoring_provider_model:
-                proctoring_provider = proctoring_provider_model.get('value')
-            else:
-                proctoring_provider = block.proctoring_provider
-
             missing_escalation_email_msg = 'Provider \'{provider}\' requires an exam escalation contact.'
             if proctoring_provider_model and proctoring_provider == 'proctortrack':
                 if not escalation_email: