Return a 404 when a block is not found.

[winder]

Summary

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

Test Plan

New unit test.

[codecov-commenter]

Codecov Report

Merging #719 (5095078) into develop (7e2eff0) will increase coverage by 0.19%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop     #719      +/-   ##
===========================================
+ Coverage    53.55%   53.75%   +0.19%     
===========================================
  Files           23       23              
  Lines         3671     3676       +5     
===========================================
+ Hits          1966     1976      +10     
+ Misses        1442     1436       -6     
- Partials       263      264       +1     

Impacted Files	Coverage Δ
idb/postgres/postgres.go	46.34% <0.00%> (-0.11%)	⬇️
api/handlers.go	14.45% <100.00%> (+2.35%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7e2eff0...5095078. Read the comment docs.