Make Ae forgiving when imported files are not accessible

[jaydude-wb]

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. 

[jenkmeister]

We can have a discussion internally about how that could be improved. For now, you can change the default automatic footage reloading setting in Import Preferences:

It would mean manually reloading the footage after the render is complete, but would avoid the error dialogs for now.

[jenkmeister]

Acknowledge for review