go/tendermint/keymanager: error Status() if keymanager doesn't exist

[ptrus]

This fixes panics in keymanager client if key manager doesn't exist.

[codecov]

Codecov Report

Merging #2628 into master will decrease coverage by 0.03%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #2628      +/-   ##
==========================================
- Coverage   63.17%   63.13%   -0.04%     
==========================================
  Files         360      360              
  Lines       34046    34048       +2     
==========================================
- Hits        21507    21497      -10     
- Misses       9879     9895      +16     
+ Partials     2660     2656       -4

Impacted Files	Coverage Δ
go/keymanager/api/api.go	66.66% <ø> (ø)	⬆️
...onsensus/tendermint/apps/keymanager/state/state.go	86.04% <100%> (ø)	⬆️
...consensus/tendermint/apps/keymanager/keymanager.go	61.74% <85.71%> (+0.52%)	⬆️
go/consensus/api/grpc.go	57.02% <0%> (-6.62%)	⬇️
go/runtime/history/prune.go	74.19% <0%> (-3.23%)	⬇️
go/storage/metrics.go	73.21% <0%> (-2.68%)	⬇️
go/storage/mkvs/urkel/lookup.go	72.72% <0%> (-2.28%)	⬇️
go/consensus/tendermint/tendermint.go	65.55% <0%> (-1.4%)	⬇️
go/runtime/transaction/transaction.go	77.1% <0%> (-1.21%)	⬇️
go/worker/compute/executor/committee/node.go	61.66% <0%> (-1.19%)	⬇️
... and 6 more

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[kostko]

👍