fix(migrations): make app prefix migration language independent

Currently, the migration to add prefixes failed on initial migrations if
the database server is not in English. This commit removes the
dependency on the error message and instead uses the underlying psycopg
exception which is more specific.

caluma/caluma_core/management/commands/migrate_to_prefixed_apps.py

@@ -1,6 +1,7 @@
 from django.core.management.base import BaseCommand
 from django.db import connection
 from django.db.utils import ProgrammingError
+from psycopg2.errors import UndefinedTable
 
 
 class Command(BaseCommand):
@@ -35,8 +36,10 @@ def _migration_needed(self):
             try:
                 cursor.execute(query, params=None)
             except ProgrammingError as e:  # pragma: no cover
-                # happens only on initial migration for new project
-                if e.args[0].startswith('relation "django_migrations" does not exist'):
+                # Happens only on initial migration for new project.
+                # `e.__cause__` contains the underlying psycopg2 exception
+                # which is more specific than the django wrapper exception
+                if isinstance(e.__cause__, UndefinedTable):
                     return False
                 raise
             if cursor.fetchone():