Add try/catch block to upgrade function until better solution is found

openlp/plugins/songs/lib/upgrade.py

@@ -104,6 +104,9 @@ def upgrade_4(session, metadata):
 
     This upgrade adds a column for author type to the authors_songs table
     """
+    try:
         op = get_upgrade_op(session)
         op.add_column('authors_songs', Column('author_type', types.String(), primary_key=True,
                                               nullable=False, server_default=text('""')))
+    except OperationalError:
+        log.info('Upgrade 4 has already been run')