Fix #9715: Simplify error message if test severity isn't 'warn' or 'error'

[aranke]

resolves #9715

Problem

Error message is unintuitive if test severity is configured incorrectly.

Solution

Raise better error before JSON schema validation

Notes

Unit test is failing, this is expected because of this commit: dbt-labs/dbt-adapters@b61d66d

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.14%. Comparing base (27943a5) to head (be2ab9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10015      +/-   ##
==========================================
- Coverage   88.17%   88.14%   -0.03%     
==========================================
  Files         181      181              
  Lines       22614    22617       +3     
==========================================
- Hits        19939    19935       -4     
- Misses       2675     2682       +7     

Flag	Coverage Δ
integration	85.44% <66.66%> (-0.13%)	⬇️
unit	62.24% <100.00%> (+0.03%)	⬆️

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