Clean up and improve error handling

[araghukas]

Updated the plugin with the following changes:

• add a new variables parameter for environment variables
• update plugin defaults to use BaseModel instead of dict
• add a new error-catching python execution script (new module)
• add checks inside submit script for covalent and cloudpickle versions
• change to actually get errors from these checks
• clean up job script creation (new module)
• use Path everywhere instead of os.path operations
• allow poll_freq >= 10 seconds, instead of 60 seconds
• misc. cleanups and refactoring

Notes

• these changes undo the previous auto-setting of COVALENT_CONFIG_DIR. In case of the underlying error, the user will instead be informed with specific instructions.

---

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

[codecov[bot]]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (f5085fb) 93.16% compared to head (429f0c6) 92.00%.

Files	Patch %	Lines
covalent_slurm_plugin/exec.py	91.48%	4 Missing :warning:
covalent_slurm_plugin/job_script.py	96.00%	3 Missing :warning:
covalent_slurm_plugin/slurm.py	97.50%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #86 +/- ## =========================================== - Coverage 93.16% 92.00% -1.17% =========================================== Files 2 4 +2 Lines 278 375 +97 =========================================== + Hits 259 345 +86 - Misses 19 30 +11 ```

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