Validate SRUN variables when launching in SLURM

[awaelchli]

What does this PR do?

Addresses suggestion in #14986, frequent user errors that result in their SLURM jobs hanging. By adding warnings and errors, we can prevent these frequent questions and give feedback directly at runtime.

Fixes #10150

No intended breaking changes:

• Lightning SLURM users were always meant to use the --ntasks-per-node, otherwise their processes would hang. Hence, the error is appropriate.
• The warning about srun not being used is very unlikely going to be a false positive, because the srun program is only installed on SLURM clusters. However, we won't make it an error just in case there is another program called srun on your system unrelated to SLURM.

Before submitting

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PR review

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @awaelchli @carmocca @justusschock

[codecov]

Codecov Report

Merging #15011 (0945cdd) into master (26f632c) will increase coverage by 2%.
The diff coverage is 96%.

❗ Current head 0945cdd differs from pull request most recent head f7a2948. Consider uploading reports for the commit f7a2948 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #15011     +/-   ##
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+ Coverage      82%      84%     +2%     
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  Files         412      288    -124     
  Lines       30265    21962   -8303     
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- Hits        24869    18494   -6375     
+ Misses       5396     3468   -1928