go/common/quantity: Avoid side-effects on errors

[Yawning]

In theory this is non-breaking because we check errors and don't use the set value.

[codecov]

Codecov Report

Merging #4790 (2a86f51) into master (c30f051) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4790      +/-   ##
==========================================
- Coverage   67.02%   66.99%   -0.04%     
==========================================
  Files         451      451              
  Lines       50295    50289       -6     
==========================================
- Hits        33711    33690      -21     
- Misses      12431    12443      +12     
- Partials     4153     4156       +3     

Impacted Files	Coverage Δ
go/common/quantity/quantity.go	94.17% <100.00%> (+3.34%)	⬆️
go/consensus/tendermint/apps/staking/votes.go	57.89% <0.00%> (-10.53%)	⬇️
go/consensus/tendermint/apps/roothash/query.go	86.20% <0.00%> (-6.90%)	⬇️
go/common/grpc/auth/auth.go	94.44% <0.00%> (-5.56%)	⬇️
go/worker/common/p2p/dispatch.go	71.52% <0.00%> (-5.56%)	⬇️
go/consensus/tendermint/full/services.go	74.81% <0.00%> (-3.71%)	⬇️
go/sentry/api/grpc.go	40.62% <0.00%> (-3.13%)	⬇️
...nsensus/tendermint/apps/keymanager/transactions.go	52.77% <0.00%> (-2.78%)	⬇️
go/consensus/tendermint/apps/staking/staking.go	56.90% <0.00%> (-2.21%)	⬇️
go/storage/mkvs/lookup.go	77.08% <0.00%> (-2.09%)	⬇️
... and 21 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 767eb0b...2a86f51. Read the comment docs.