fix(p2p/exchange): Remove peer blocking from head request

[renaynay]

Currently, the exchange will block peers for giving a bad header that does not pass the Verify check (unless it is a SoftFailure). Unless we introduce another field inside of VerifyError to indicate to the caller whether the error was severe enough to warrant blocking, we shouldn't blanket block for verify errors as a good peer that sends a slightly outdated header will get banned.

(this is currently observable on arabica - have not tested on mocha yet)

Moral of the story: do not prematurely implement blocking behaviour.

[codecov-commenter]

Codecov Report

Merging #113 (a2895a7) into main (e75d00d) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   69.20%   69.16%   -0.05%     
==========================================
  Files          37       37              
  Lines        2939     2938       -1     
==========================================
- Hits         2034     2032       -2     
  Misses        756      756              
- Partials      149      150       +1     

Files	Coverage Δ
p2p/exchange.go	85.89% <ø> (-0.07%)	⬇️

... and 2 files with indirect coverage changes