Make Opacity required component for Lifespan.

[tejaboy]

There was no indication that the lifespan component requires the opacity component. I spent quite sometime debugging it for my own game. It was when I used the fadeIn component, then the error state that the opacity component was required. So I thought it'd be nice that lifespan component gives an error like fadeIn.

When the error does not prevent the script from continuing, I added a shorthand code to set the opacity to 1 if it's not set, else the current opacity.