Raise alarm for 'still marked as enabled' modules

The module shows 'Previously installed' modules. That's of nice informational value, but it can be any odd module which I enabled, disabled and uninstalled.

However, I REALLY want to see modules stand out that are still marked 'enabled' in the system table.
That can be pretty handy when moving a database to another server with another file system. (If I forgot to copy an enabled module over, I do _not_ want to accidentally delete that line from the systems table.)

Since I wanted a higher warning level, I used the 'error.png' icon.