[MICROBA-1139]

* Change order of object Manager declarations in the GeneratedCertificate model. Declaration order matters and It appears that having the "base" `objects` declaration after the other Custom Managers creates an issue where I cannot access `objects` in a migration without this change.
* As part of the manager migration there is a cleanup function that resets the `error_reason` field to an empty string for all certificates in the `downloadable` state that currently have an error listed in the record.

lms/djangoapps/certificates/migrations/0025_cleanup_certificate_errors.py

@@ -0,0 +1,30 @@
+# Generated by Django 2.2.20 on 2021-04-27 18:12
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    """
+    Clean records in the `CERTIFICATES_GENERATEDCERTIFICATE` table that are in the downloadable state but also have
+    old errors still part of their certificate record.
+
+    As part of this migration we are also altering the Managers for the GeneratedCertificate model. We need the ability
+    to access the `objects` attribute for the cleanup.
+    """
+    def cleanup_certificate_records(apps, schema_editor):
+        GeneratedCertificate = apps.get_model('certificates', 'GeneratedCertificate')
+
+        GeneratedCertificate.objects.filter(status='downloadable').exclude(error_reason='').update(error_reason='')
+
+    dependencies = [
+        ('certificates', '0024_delete_allowlistgenerationconfiguration'),
+    ]
+
+    operations = [
+        migrations.AlterModelManagers(
+            name='generatedcertificate',
+            managers=[
+            ],
+        ),
+        migrations.RunPython(cleanup_certificate_records, reverse_code=migrations.RunPython.noop)
+    ]

lms/djangoapps/certificates/models.py

@@ -202,7 +202,7 @@ class EligibleAvailableCertificateManager(EligibleCertificateManager):
     A manager for `GeneratedCertificate` models that automatically
     filters out ineligible certs and any linked to nonexistent courses.
 
-    Adds to the super class filtering ot also exclude certificates for
+    Adds to the super class filtering to also exclude certificates for
     courses that do not have a corresponding CourseOverview.
     """
 
@@ -233,6 +233,11 @@ class GeneratedCertificate(models.Model):
     # the course_modes app is loaded, resulting in a Django deprecation warning.
     from common.djangoapps.course_modes.models import CourseMode  # pylint: disable=reimported
 
+    # Normal object manager, which should only be used when ineligible
+    # certificates (i.e. new audit certs) should be included in the
+    # results. Django requires us to explicitly declare this.
+    objects = models.Manager()
+
     # Only returns eligible certificates. This should be used in
     # preference to the default `objects` manager in most cases.
     eligible_certificates = EligibleCertificateManager()
@@ -241,11 +246,6 @@ class GeneratedCertificate(models.Model):
     # associated CourseOverview
     eligible_available_certificates = EligibleAvailableCertificateManager()
 
-    # Normal object manager, which should only be used when ineligible
-    # certificates (i.e. new audit certs) should be included in the
-    # results. Django requires us to explicitly declare this.
-    objects = models.Manager()
-
     MODES = Choices(
         'verified',
         'honor',