Check returncode before assuming task has failed because stderr is non-empty

[araghukas]

• I have added the tests to cover my changes.
• I have updated the documentation, VERSION, and CHANGELOG accordingly.
• I have read the CONTRIBUTING document.

[codecov]

Codecov Report

Base: 50.00% // Head: 49.68% // Decreases project coverage by -0.32% ⚠️

Coverage data is based on head (c90e6c8) compared to base (dcd5fc2).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #62      +/-   ##
===========================================
- Coverage    50.00%   49.68%   -0.32%     
===========================================
  Files            2        2              
  Lines          160      161       +1     
===========================================
  Hits            80       80              
- Misses          80       81       +1     

Impacted Files	Coverage Δ
covalent_ssh_plugin/ssh.py	49.37% <0.00%> (-0.32%)	⬇️

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