Viewer should show message for off-line file instead of blank/black screen

[ubacher]

Currently (1.44) the viewer shows a blank/black screen when a (non-cached) jpg file is off-line.
I would suggest it should display a message:

Image can not bee shown because it is off-line and does not have a cache image

Will avoid some confusion.

[Mario]

Implemented for build 5.0.146