Request #30111:  Debugger pops up dialog when source not available.

Load an object file, which has no associated source file, and which
also includes a function that raises an exception.  Call the function
to raise the exception.  The debugger should appear giving no
indication that the source file was not found, ie. it fails silently.
Double click on an item on the stack.  A dialog should then pop up 
indicating that the source file could not be read.  If the file viewer
was already displayed it is blanked out, otherwise it is not displayed
at all.  Also included in this change:  double click on an item in the
History tool, and the File Viewer should pop up if not already visible
and display the source of the selection (same as choosing
Usage->ViewSource).
