Show a message to the user stating ARGB images cannot be processed.

[tinevez]

Right now, if you feed TrackMate with an ARGB or a RGB image, it generates a class cast exception.
Would it be feasible to show a friendly dialog informing the user of such, rather than emitting the ClassCastException?

I seem to remember there was such a dialog in a past version of TrackMate, I wonder where it has gone. So, this is a bug.