cli/server: don't spit out error if notary request already exists

[roman-khimov]

It could be sent by another actor and that's OK.

Although signatures from this request should still be processed.

[codecov]

Codecov Report

Merging #2426 (2c16e04) into master (c12a3b7) will decrease coverage by 0.02%.
The diff coverage is 91.91%.

@@            Coverage Diff             @@
##           master    #2426      +/-   ##
==========================================
- Coverage   85.14%   85.11%   -0.03%     
==========================================
  Files         290      289       -1     
  Lines       36139    36035     -104     
==========================================
- Hits        30770    30672      -98     
+ Misses       4088     4082       -6     
  Partials     1281     1281              

Impacted Files	Coverage Δ
cli/server/server.go	79.78% <0.00%> (-0.17%)	⬇️
cli/smartcontract/generate.go	83.33% <ø> (ø)
pkg/core/blockchain.go	83.02% <50.00%> (ø)
pkg/core/native/oracle.go	72.19% <71.42%> (ø)
pkg/neotest/basic.go	94.09% <92.00%> (ø)
pkg/neotest/client.go	90.00% <92.30%> (ø)
pkg/core/native/interop.go	63.23% <100.00%> (ø)
pkg/core/native/management.go	92.92% <100.00%> (ø)
pkg/neotest/chain/chain.go	94.39% <100.00%> (ø)
pkg/neotest/compile.go	100.00% <100.00%> (ø)
... and 15 more

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