Update of tables deletion process

I now delete the django tables only if they exist. It avoids very nasty
error messages that can compromise the script running.

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>

django/tools/migration.sh

@@ -41,7 +41,7 @@ DeleteDjangoTables () {
   for TABLE in $DJANGO_TABLES
   do
     mysql -u $USER $PASS -h $HOST $DB -e \
-      "SET FOREIGN_KEY_CHECKS = 0; DROP TABLE $TABLE; SET FOREIGN_KEY_CHECKS = 1;"
+      "SET FOREIGN_KEY_CHECKS = 0; DROP TABLE IF EXISTS $TABLE; SET FOREIGN_KEY_CHECKS = 1;"
   done
 }