fix(gno.land): refine error message for addpkg operation

[ltzmaxwell]

This PR introduces PkgExistError to replace InvalidPkgPathError for situations where a package already exists during the addpkg operation, enhancing clarity and specificity.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.23%. Comparing base (8bd07c9) to head (a4b416d).
Report is 37 commits behind head on master.

Files with missing lines	Patch %	Lines
gno.land/pkg/sdk/vm/errors.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2409      +/-   ##
==========================================
+ Coverage   60.21%   60.23%   +0.01%     
==========================================
  Files         561      561              
  Lines       75071    75074       +3     
==========================================
+ Hits        45205    45218      +13     
+ Misses      26485    26480       -5     
+ Partials     3381     3376       -5     

Flag	Coverage Δ
contribs/gnodev	61.40% <ø> (ø)
contribs/gnofaucet	15.31% <ø> (ø)
gno.land	64.70% <75.00%> (-0.05%)	⬇️
gnovm	64.30% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (ø)
tm2	62.10% <ø> (+0.09%)	⬆️

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[leohhhn]

What are the cases where addpkg can fail? I'm wondering if there are other cases, such as an empty file (I guess it's caught by msg.Package.Validate()), or other cases? What if we have two files with identical names in the same package?

Just running thoughts, we should clearly define the failure cases and improve more of these errors. It might be out of scope for this PR, but it would be great to have an analysis of this. Approving.