go/consensus/tendermint: Correctly propagate errors

[kostko]

Not propagating the state unavailable error could lead to corruption when the
database becomes unavailable (e.g., due to running out of space or file
descriptors).

[codecov]

Codecov Report

Merging #4110 (f58415f) into master (e85fcca) will decrease coverage by 0.58%.
The diff coverage is 68.00%.

@@            Coverage Diff             @@
##           master    #4110      +/-   ##
==========================================
- Coverage   67.09%   66.51%   -0.59%     
==========================================
  Files         410      410              
  Lines       42380    42391      +11     
==========================================
- Hits        28436    28195     -241     
- Misses       9964    10211     +247     
- Partials     3980     3985       +5     

Impacted Files	Coverage Δ
go/consensus/tendermint/apps/staking/staking.go	52.79% <40.00%> (-0.08%)	⬇️
go/consensus/tendermint/apps/staking/votes.go	88.88% <71.42%> (-11.12%)	⬇️
...ensus/tendermint/apps/staking/proposing_rewards.go	83.78% <100.00%> (+0.92%)	⬆️
...nsensus/tendermint/apps/keymanager/transactions.go	0.00% <0.00%> (-52.95%)	⬇️
go/ias/http/http.go	18.51% <0.00%> (-44.45%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/keymanager/api/policy_sgx.go	23.07% <0.00%> (-15.39%)	⬇️
go/runtime/host/sandbox/sandbox.go	70.54% <0.00%> (-11.25%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
... and 39 more

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