slurm plugin: always raise for non-zero exit code

[ltalirz]

potentially fixes #4326

Prior to this change, cases where squeue would return a non-zero exit
code but an empty stderr would not lead to a SchedulerError.
This is fixed by adapting the slurm joblist command such that it is
always expected to produce exit code zero, and raising whenever a
non-zero exit code is encountered.

[codecov]

Codecov Report

Merging #4332 into develop will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #4332      +/-   ##
===========================================
+ Coverage    79.10%   79.14%   +0.05%     
===========================================
  Files          468      468              
  Lines        34614    34616       +2     
===========================================
+ Hits         27378    27395      +17     
+ Misses        7236     7221      -15     

Flag	Coverage Δ
#django	72.77% <100.00%> (+0.05%)	⬆️
#sqlalchemy	71.95% <100.00%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
aiida/schedulers/plugins/slurm.py	58.94% <100.00%> (+6.45%)	⬆️
aiida/transports/plugins/local.py	81.29% <0.00%> (-0.25%)	⬇️

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[giovannipizzi]

Some textual changes suggested

[giovannipizzi]

Thanks!
For me it could be merged, but as pointed by codecov, the new lines are not tested. Would you be willing to add one (or two?) new tests to https://github.com/aiidateam/aiida-core/blob/develop/tests/schedulers/test_slurm.py to cover the new code?

[ltalirz]

@giovannipizzi fair point - added the test, thanks for pointing out where it should go (and indeed there was still as slight mistake)

[giovannipizzi]

Thanks!