fix: Fix confusing error message on duplicate row_index

[nameexhaustion]

# Before
polars.exceptions.SchemaError: extra column in file outside of expected schema: index
# After
polars.exceptions.DuplicateError: cannot add row_index with name 'index': column already exists in file.

[codecov]

Codecov Report

Attention: Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.41%. Comparing base (fb9a4e8) to head (842bde0).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/conversion/scans.rs	96.55%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23043   +/-   ##
=======================================
  Coverage   80.41%   80.41%           
=======================================
  Files        1682     1682           
  Lines      223305   223324   +19     
  Branches     2802     2802           
=======================================
+ Hits       179572   179588   +16     
- Misses      43071    43073    +2     
- Partials      662      663    +1     

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