enhancement: Return a 404 when a block is not found.

[shiqizng]

Summary

Return a 404 instead of 500 when /v2/blocks/{round} fails due to a missing block.

Test Plan

Update handler test

[codecov]

Codecov Report

Merging #4479 (ffda15d) into master (e4d6d42) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4479      +/-   ##
==========================================
- Coverage   55.19%   55.16%   -0.03%     
==========================================
  Files         398      398              
  Lines       50165    50173       +8     
==========================================
- Hits        27689    27679      -10     
- Misses      20159    20177      +18     
  Partials     2317     2317              

Impacted Files	Coverage Δ
daemon/algod/api/server/v2/handlers.go	0.00% <0.00%> (ø)
util/db/dbutil.go	44.24% <0.00%> (-4.25%)	⬇️
ledger/acctupdates.go	69.29% <0.00%> (-0.60%)	⬇️
network/wsNetwork.go	64.63% <0.00%> (-0.20%)	⬇️
catchup/service.go	69.62% <0.00%> (+0.24%)	⬆️
catchup/peerSelector.go	100.00% <0.00%> (+1.04%)	⬆️

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[algorandskiy]

LGTM but technically this is an interface change.
Plus, the summary line/PR title should be rest api:/daemon:/xyz instead of enhancement