[cmake] Print out errors from failed stack steps
[cmake] Print out errors from failed stack steps

Per [cmake docs](https://cmake.org/cmake/help/v3.0/command/execute_process.html), if you set both `OUTPUT_VARIABLE` and `ERROR_VARIABLE` to the same var, it will contain both streams' lines, merged in the order they were printed.
tstein/bond,chwarr/bond,sapek/bond,tstein/bond,chwarr/bond,tstein/bond,jdubrule/bond,Microsoft/bond,tstein/bond,Microsoft/bond,tstein/bond,chwarr/bond,Microsoft/bond,Microsoft/bond,sapek/bond,jdubrule/bond,tstein/bond,jdubrule/bond,sapek/bond,sapek/bond,Microsoft/bond,jdubrule/bond,sapek/bond,chwarr/bond,sapek/bond,chwarr/bond,chwarr/bond,jdubrule/bond,jdubrule/bond,Microsoft/bond