Fixes error handling when reading QueryConditions
Fixes error handling when reading QueryConditions

QueryCondition#translate_to_python() now raises ValueError instead of
returning None.

minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,VisTrails/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,hjanime/VisTrails