Add error message when not running inside SLURM

[adamltyson]

Closes #8

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (95dd05f) 90.90% compared to head (3ccdec6) 92.00%.

Files	Patch %	Lines
slurmio/slurmio.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   90.90%   92.00%   +1.09%     
==========================================
  Files           2        2              
  Lines          66       75       +9     
==========================================
+ Hits           60       69       +9     
  Misses          6        6              

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[JoeZiminski]

Works great! but codecov is unhappy and I'm not certain what to do about it.