solo machine GetPubKey returns an error instead of panicing

[colin-axner]

Description

closes: #7823

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• 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 #7883 (5f70bd5) into master (0bd4657) will increase coverage by 0.00%.
The diff coverage is 66.10%.

@@           Coverage Diff           @@
##           master    #7883   +/-   ##
=======================================
  Coverage   54.20%   54.21%           
=======================================
  Files         610      610           
  Lines       38847    38868   +21     
=======================================
+ Hits        21056    21071   +15     
- Misses      15636    15639    +3     
- Partials     2155     2158    +3     

[fedekunze]

changes LGTM. Pending a few unit test cases

[fedekunze]

utACK

[cwgoes]

ACK