fix: Use f-string to show filename in error message correctly

[kdheepak]

This fix makes the string an f-string. Without this change, the error message doesn't show which file is actually missing:

[kdheepak]

I also think this should be a FileNotFoundError exception instead. Let me know if you'd like me to submit a separate PR for that.

[philippjfr]

I also think this should be a FileNotFoundError exception instead. Let me know if you'd like me to submit a separate PR for that.

Yes, I think so. Also investigating what's up with the test suite, must have regressed somehow over the holidays.

[codecov]

Codecov Report

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

Project coverage is 82.10%. Comparing base (400cc44) to head (66a3a46).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/command/convert.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7589      +/-   ##
==========================================
- Coverage   82.11%   82.10%   -0.01%     
==========================================
  Files         340      340              
  Lines       51477    51477              
==========================================
- Hits        42271    42266       -5     
- Misses       9206     9211       +5     

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[philippjfr]

Thank you!