Return RPC error if getBlock's lowHash doesn't exist

[someone235]

No description provided.

[codecov]

Codecov Report

Merging #1569 (f8ea19e) into v0.9.0-dev (ce326b3) will decrease coverage by 0.04%.
The diff coverage is 14.28%.

@@              Coverage Diff               @@
##           v0.9.0-dev    #1569      +/-   ##
==============================================
- Coverage       61.49%   61.45%   -0.05%     
==============================================
  Files             514      514              
  Lines           19687    19694       +7     
==============================================
- Hits            12107    12102       -5     
- Misses           5818     5826       +8     
- Partials         1762     1766       +4     

Impacted Files	Coverage Δ
app/rpc/rpchandlers/get_blocks.go	32.65% <14.28%> (-3.07%)	⬇️
...k/netadapter/server/grpcserver/connection_loops.go	53.84% <0.00%> (-3.85%)	⬇️
infrastructure/network/addressmanager/store.go	77.06% <0.00%> (-1.84%)	⬇️
app/protocol/flows/blockrelay/handle_relay_invs.go	68.75% <0.00%> (-1.25%)	⬇️

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