Fail early if $SCRATCH doesn't exist @ NERSC

[MrCreosote]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 56.04%. Comparing base (e44319e) to head (788235e).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cdmtaskservice/nersc/manager.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
+ Coverage   56.00%   56.04%   +0.04%     
==========================================
  Files          29       30       +1     
  Lines        1764     1777      +13     
==========================================
+ Hits          988      996       +8     
- Misses        776      781       +5     

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