Improved error message for execute app.
Improved error message for execute app.

The error message now includes the exception type as well as a full traceback at the Debug level.

ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,goddardl/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,chippey/gaffer,hradec/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,chippey/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,boberfly/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,hradec/gaffer,hradec/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,hradec/gaffer,goddardl/gaffer,chippey/gaffer,boberfly/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,boberfly/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,chippey/gaffer,boberfly/gaffer,ImageEngine/gaffer,goddardl/gaffer,johnhaddon/gaffer,goddardl/gaffer