stereo_gui: Clarify error message
stereo_gui: Clarify error message

oleg-alexandrov/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,oleg-alexandrov/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,oleg-alexandrov/StereoPipeline