fix: Standardised additional SQL interface errors

[alexander-beedie]

• (fix) SQL parser errors should be mapped to the new SQLInterface error (not ComputeError).
• (misc) Standardised use of "can't" vs "cannot" in error messages.
• (refactor) Consolidated some test files.

[codecov]

Codecov Report

Attention: Patch coverage is 52.38095% with 10 lines in your changes missing coverage. Please review.

Project coverage is 81.39%. Comparing base (f548041) to head (c109a92).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-sql/src/functions.rs	11.11%	8 Missing ⚠️
crates/polars-sql/src/context.rs	66.66%	1 Missing ⚠️
crates/polars-sql/src/sql_expr.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16829   +/-   ##
=======================================
  Coverage   81.38%   81.39%           
=======================================
  Files        1425     1425           
  Lines      187610   187615    +5     
  Branches     2697     2697           
=======================================
+ Hits       152695   152700    +5     
  Misses      34420    34420           
  Partials      495      495           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.