update SigVerifiableTx.GetPubKeys to return an error

[robert-zaremba]

Description

closes: #8129

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• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #8828 (ec10c1e) into master (24ed401) will decrease coverage by 0.01%.
The diff coverage is 46.66%.

@@            Coverage Diff             @@
##           master    #8828      +/-   ##
==========================================
- Coverage   59.45%   59.44%   -0.02%     
==========================================
  Files         563      563              
  Lines       31277    31286       +9     
==========================================
+ Hits        18597    18598       +1     
- Misses      10518    10522       +4     
- Partials     2162     2166       +4     

Impacted Files	Coverage Δ
x/auth/ante/sigverify.go	73.18% <33.33%> (-1.68%)	⬇️
x/auth/tx/builder.go	74.24% <50.00%> (-2.14%)	⬇️
x/auth/legacy/legacytx/stdtx.go	80.85% <100.00%> (ø)