Make Ae forgiving when imported files are not accessible

When compositing Comps with 3d rendered elements, this can bring in a situation where an already imported render needs adjustments in the 3d app and then it has to be rendered again. While this is rendering, Ae panics, and comes up with all kinds of errors, while it just should wait for the render to finish.

Why not do it the Premiere way - just say that the footage is offline, when it is not accessible, or show the Layer name in Italics or grayed out.