go/staking/api: Fix genesis sanity check for nonexisting accounts

[tjanez]

Detect when a (debonding) delegation is specified for a nonexisting account and report an appropriate error.

[codecov]

Codecov Report

Merging #2671 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2671      +/-   ##
==========================================
- Coverage   63.07%   63.04%   -0.03%     
==========================================
  Files         365      365              
  Lines       34586    34592       +6     
==========================================
- Hits        21815    21810       -5     
- Misses      10060    10063       +3     
- Partials     2711     2719       +8     

Impacted Files	Coverage Δ
go/worker/storage/service_external.go	47.19% <0.00%> (-4.50%)	⬇️
go/runtime/tagindexer/tagindexer.go	68.47% <0.00%> (-4.35%)	⬇️
go/runtime/transaction/transaction.go	74.69% <0.00%> (-3.62%)	⬇️
go/oasis-node/cmd/debug/storage/storage.go	45.08% <0.00%> (-3.47%)	⬇️
go/worker/keymanager/worker.go	60.00% <0.00%> (-2.99%)	⬇️
go/worker/common/p2p/p2p.go	64.86% <0.00%> (-2.71%)	⬇️
go/storage/mkvs/urkel/iterator.go	82.46% <0.00%> (-2.60%)	⬇️
go/storage/client/client.go	74.78% <0.00%> (-1.74%)	⬇️
go/storage/mkvs/urkel/cache.go	81.27% <0.00%> (-1.71%)	⬇️
go/worker/compute/executor/committee/node.go	61.66% <0.00%> (-1.19%)	⬇️
... and 11 more

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