tests:image:rgb2greyscale: Use exit code to report errors
tests:image:rgb2greyscale: Use exit code to report errors

So 'make check' can pick it up.
Reported in #175

Signed-off-by: Ola Jeppsson <793f970c52ded1276b9264c742f19d1888cbaf73@adapteva.com>

eliteraspberries/pal,eliteraspberries/pal,mateunho/pal,parallella/pal,aolofsson/pal,Adamszk/pal3,olajep/pal,8l/pal,eliteraspberries/pal,aolofsson/pal,mateunho/pal,eliteraspberries/pal,debug-de-su-ka/pal,Adamszk/pal3,debug-de-su-ka/pal,olajep/pal,mateunho/pal,8l/pal,Adamszk/pal3,aolofsson/pal,parallella/pal,parallella/pal,parallella/pal,aolofsson/pal,olajep/pal,olajep/pal,8l/pal,debug-de-su-ka/pal,mateunho/pal,parallella/pal,eliteraspberries/pal,debug-de-su-ka/pal,debug-de-su-ka/pal,Adamszk/pal3,8l/pal,mateunho/pal