Avoid crashing when stress is not implemented

[giovannipizzi]

In QE <= 6.5, stress could not computed when using non collinear magnetism (spin-orbit) and GGA functionals. The message in the output would read:

     Message from routine stres:
     noncollinear stress + GGA not implemented

The parser would then completely crash and leave the calculation in an Excepted state, with a traceback like the following:

*** 3 LOG MESSAGES:
+-> WARNING at 2020-09-22 14:50:17.398914+02:00
 | could not parse scheduler output: output parsing is not available for `SlurmScheduler`
+-> WARNING at 2020-09-22 14:50:18.021878+02:00
 | Error while parsing stress tensor: "P=" not found within 15 lines from the start of the stress block
+-> REPORT at 2020-09-22 14:50:18.138142+02:00
 | [4004|PwCalculation|on_except]: Traceback (most recent call last):
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/utils/validation/trajectory.py", line 84, in verify_convergence_stress
 |     stress = trajectory.get_array('stress')[index]
 |   File "/home/pizzi/git/aiida-core/aiida/orm/nodes/data/array/array.py", line 122, in get_array
 |     return get_array_from_file(self, name)
 |   File "/home/pizzi/git/aiida-core/aiida/orm/nodes/data/array/array.py", line 114, in get_array_from_file
 |     raise KeyError('Array with name `{}` not found in ArrayData<{}>'.format(name, self.pk))
 | KeyError: 'Array with name `stress` not found in ArrayData<None>'
 | 
 | During handling of the above exception, another exception occurred:
 | 
 | Traceback (most recent call last):
 |   File "/home/pizzi/.virtualenvs/aiida-dev/lib/python3.7/site-packages/plumpy/process_states.py", line 225, in execute
 |     result = self.run_fn(*self.args, **self.kwargs)
 |   File "/home/pizzi/git/aiida-core/aiida/engine/processes/calcjobs/calcjob.py", line 313, in parse
 |     exit_code_retrieved = self.parse_retrieved_output(retrieved_temporary_folder)
 |   File "/home/pizzi/git/aiida-core/aiida/engine/processes/calcjobs/calcjob.py", line 387, in parse_retrieved_output
 |     exit_code = parser.parse(**parse_kwargs)
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/parsers/pw.py", line 128, in parse
 |     exit_code = validator(trajectory, parsed_parameters, logs_stdout)
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/parsers/pw.py", line 206, in validate_ionic
 |     if not self.is_ionically_converged(trajectory):
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/parsers/pw.py", line 241, in is_ionically_converged
 |     converged_relax = verify_convergence_trajectory(trajectory, -2, *values)
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/utils/validation/trajectory.py", line 33, in verify_convergence_trajectory
 |     converged_stress = verify_convergence_stress(trajectory, index, threshold_stress, reference_pressure)
 |   File "/home/pizzi/git/plugins_aiida/aiida-quantumespresso/aiida_quantumespresso/utils/validation/trajectory.py", line 86, in verify_convergence_stress
 |     raise ValueError('the `stress` array does not exist or the given index exceeds the length.')
 | ValueError: the `stress` array does not exist or the given index exceeds the length.

A few notes:

• QE 6.6 now supports these simulations, so this error would not be obtained in QE6.6. Still, there are (I'm sure) many more combinations of features and functionals that are not implemented, so this issue still remains.
• the error does not occur when using relax only as the current plugin code correctly skips the checks on the stress in that cae
• What QE does in this case (as far as I understand) is, beyond printing the error message shown earlier, to just ignore the stress (i.e. it performs a 'relax' instead of a 'vc-relax')
• I think that the correct behaviour in this case would be not to except, but to return a proper (new) Exit Code that says "look, you asked for a vc-relax, but QE just did a relax because of the stress not being implemented with your specific combination of inputs".

[sphuber]

After discussion, we agree that it should most likely have an 5** exit code, since it's relax/vc-relax related and the PwRelaxWorkChain should add a handler for this that when vc-relax is used as a mode, it is stopped.