core: improve ambiguous block validation message

[ucwong]

No description provided.

[codecov]

Codecov Report

Merging #1417 (57594a4) into master (8b5105a) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1417      +/-   ##
==========================================
+ Coverage   51.96%   51.98%   +0.01%     
==========================================
  Files         464      464              
  Lines       58429    58429              
==========================================
+ Hits        30365    30375      +10     
+ Misses      25755    25743      -12     
- Partials     2309     2311       +2     

Impacted Files	Coverage Δ
core/block_validator.go	42.85% <0.00%> (ø)
rpc/server.go	76.27% <0.00%> (-6.78%)	⬇️
p2p/discover/table.go	84.19% <0.00%> (-2.44%)	⬇️
core/rawdb/chain_iterator.go	60.89% <0.00%> (-1.12%)	⬇️
trie/proof.go	75.45% <0.00%> (-0.91%)	⬇️
p2p/discover/v5_udp.go	81.14% <0.00%> (-0.44%)	⬇️
ctxc/downloader/downloader.go	77.51% <0.00%> (ø)
p2p/server.go	66.35% <0.00%> (+0.37%)	⬆️
ctxc/downloader/queue.go	80.46% <0.00%> (+0.46%)	⬆️
rpc/client.go	86.42% <0.00%> (+0.99%)	⬆️
... and 5 more