Notify user when loading a sequence which contains missing elements

[GoogleCodeExporter]

When a sequence loads onto the stage, it currently skips any references 
expressions/sequences that it can't find on disk (a message is logged to the 
console, but that's it).  It would probably be nice to notify the user so that 
she knows that something is missing. Maybe even render the missing 
expression/sequence on the stage, but in red to make it clear that it's missing.

Original issue reported on code.google.com by garthabr...@gmail.com on 13 Jul 2011 at 8:59

• Blocked on: Need UI elements for "ghost" elements for displaying a sequence which contains missing elements #100