Improve error message returned from AddSignatureFromPubKey

[alexanderbez]

Improve error message returned from AddSignatureFromPubKey and cleanup a unit test. I don't think this is worthy of a pending log but I'd be happy to add one if desired.

closes: #3672

---

• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG_PENDING.md

[codecov-io]

Codecov Report

Merging #3675 into develop will decrease coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           develop    #3675     +/-   ##
==========================================
- Coverage    63.34%   63.25%   -0.1%     
==========================================
  Files          218      218             
  Lines        18205    18208      +3     
==========================================
- Hits         11532    11517     -15     
- Misses        5711     5722     +11     
- Partials       962      969      +7

Impacted Files	Coverage Δ
crypto/multisig/multisignature.go	70% <0%> (-7.78%)	⬇️
consensus/ticker.go	91.66% <0%> (-4.17%)	⬇️
privval/signer_service_endpoint.go	85.45% <0%> (-3.64%)	⬇️
consensus/reactor.go	71.19% <0%> (-1.54%)	⬇️
blockchain/reactor.go	71.49% <0%> (-0.94%)	⬇️
consensus/state.go	79.73% <0%> (-0.48%)	⬇️
privval/signer_validator_endpoint.go	85.55% <0%> (ø)	⬆️
consensus/replay.go	71.02% <0%> (+0.81%)	⬆️
p2p/pex/pex_reactor.go	82.64% <0%> (+1.17%)	⬆️
privval/socket_listeners.go	89.65% <0%> (+3.44%)	⬆️

[melekes]

🌸

[liamsi]

👍