fixup! Implement the tracing mechanism

This is a fixup for commit 3aded8e0.

my2to3/trace.py

@@ -29,7 +29,7 @@ def create_table(table, *values):
       c.execute('INSERT INTO %s VALUES (%s)' % (
         table, ', '.join(['?'] * len(args))), args)
     except sqlite3.IntegrityError as e:
-      if not e.message.startswith('UNIQUE constraint failed:'):
+      if not str(e).startswith('UNIQUE constraint failed:'):
         raise
     else:
       conn.commit()