fix: Replace unreachable with polars_bail

[jqnatividad]

as its actually reachable and causes a panic when attempting unsupported ops like INSERT, UPDATE, DELETE, etc.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.77%. Comparing base (d9f3e3d) to head (6a3796d).

Files with missing lines	Patch %	Lines
crates/polars-sql/src/context.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20789      +/-   ##
==========================================
- Coverage   79.78%   79.77%   -0.01%     
==========================================
  Files        1561     1561              
  Lines      221982   221982              
  Branches     2531     2531              
==========================================
- Hits       177107   177097      -10     
- Misses      44293    44303      +10     
  Partials      582      582              

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[ritchie46]

Nice, could you add a test on the python side for this? E.g. testing you get the error you expect.