fix(generated_column): correct the output message when use impure expr in generated columns.

[yuhao-su]

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What's changed and what's your intention?

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
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[codecov]

Codecov Report

Merging #12494 (1251722) into main (063f58e) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #12494      +/-   ##
==========================================
+ Coverage   69.41%   69.43%   +0.01%     
==========================================
  Files        1429     1429              
  Lines      238590   238594       +4     
==========================================
+ Hits       165621   165664      +43     
+ Misses      72969    72930      -39     

Flag	Coverage Δ
rust	69.43% <0.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/frontend/src/handler/create_table.rs	84.15% <0.00%> (-0.47%)	⬇️

... and 7 files with indirect coverage changes

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