Nexus doesn't detect run failure

**Describe the bug**
Running `lab2_qmc_basics/oxygen_dimer` without QE binaries on the PATH.
Modify `machine` to `ws8`.
```
$ python O_dimer.py
...
starting runs:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
elapsed time 0.0 s memory 105.95 MB
Entering ./scale_1.0 0
writing input files 0 dft
Entering ./scale_1.0 0
sending required files 0 dft
submitting job 0 dft
Entering ./scale_1.0 0
Executing:
export OMP_NUM_THREADS=1
mpirun -np 4 pw.x -input dft.in

elapsed time 3.0 s memory 105.98 MB
elapsed time 6.1 s memory 105.99 MB
elapsed time 9.1 s memory 105.99 MB
```
No running pw.x and no error caught. nexus just sits idle.

Then `export PATH=my_qe_installation/bin:$PATH`
```
$ python O_dimer.py
...
starting runs:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
elapsed time 0.0 s memory 105.98 MB
elapsed time 3.0 s memory 105.98 MB
elapsed time 6.1 s memory 105.99 MB
elapsed time 9.1 s memory 105.99 MB
elapsed time 12.1 s memory 105.99 MB
```
pw.x still doesn't run and no error caught. nexus just sits idle.
```
$ rm scale_1.0
$ python O_dimer.py
...
starting runs:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
elapsed time 0.0 s memory 106.28 MB
Entering ./scale_1.0 0
writing input files 0 dft
Entering ./scale_1.0 0
sending required files 0 dft
submitting job 0 dft
Entering ./scale_1.0 0
Executing:
export OMP_NUM_THREADS=1
mpirun -np 4 pw.x -input dft.in

elapsed time 3.0 s memory 577.72 MB
elapsed time 6.1 s memory 2247.41 MB
elapsed time 9.1 s memory 2919.17 MB
elapsed time 12.2 s memory 3127.30 MB
```
finally it runs. then next is qmcpack run
```
Entering ./scale_1.0 2
writing input files 2 opt
Entering ./scale_1.0 2
sending required files 2 opt
submitting job 2 opt
Entering ./scale_1.0 2
Executing:
export OMP_NUM_THREADS=1
mpirun -np 4 qmcpack opt.in.xml

elapsed time 134.7 s memory 106.50 MB
elapsed time 137.7 s memory 106.51 MB
elapsed time 140.8 s memory 106.51 MB
elapsed time 143.8 s memory 106.51 MB
```
again it is idle again dule to missing qmcpack executables on the PATH.

**Expected behavior**
Properly error out when mpirun fails. Probably also worth checking the existence of executables.

**System:**
- laptop ws8