catchup: Provide more information to client when requested block not available

[cce]

Summary

When a node requests a block that is outside the range that a node contains, it returns a 404 (for HTTP catchup) or an {ErrorKey: blockNotAvailableErrMsg} value (for websockets catchup). This provides more information to the client so it can more effectively request and poll for new blocks.

Test Plan

Updated TestUGetBlockWs and TestUGetBlockHTTP.

[codecov]

Codecov Report

Merging #5819 (623d4f1) into master (c4e94fe) will increase coverage by 0.01%.
Report is 17 commits behind head on master.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #5819      +/-   ##
==========================================
+ Coverage   55.57%   55.59%   +0.01%     
==========================================
  Files         475      475              
  Lines       66838    66850      +12     
==========================================
+ Hits        37147    37163      +16     
  Misses      27172    27172              
+ Partials     2519     2515       -4     

Files	Coverage Δ
catchup/universalFetcher.go	95.26% <100.00%> (-0.37%)	⬇️
rpcs/blockService.go	73.33% <28.57%> (-1.01%)	⬇️

... and 7 files with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!