feat: Raise informative error message when join would introduce duplicate column name

[MarcoGorelli]

closes #15035

The example in that issue would now show:

polars.exceptions.DuplicateError: column with name 'c_right' already exists.

You may want to try:
- renaming the column prior to joining
- using `suffix` to specify a suffix different to the default one (`_right`)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.01%. Comparing base (bbaf341) to head (b9f45e3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15042      +/-   ##
==========================================
- Coverage   81.01%   81.01%   -0.01%     
==========================================
  Files        1338     1338              
  Lines      173601   173601              
  Branches     2460     2461       +1     
==========================================
- Hits       140645   140635      -10     
- Misses      32487    32496       +9     
- Partials      469      470       +1     

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[stinodego]

Good improvement! Minor nitpicks on the message.

[ritchie46]

Really nice one @MarcoGorelli. I took the liberty to merge @stinodego's suggestions.