bug(sync): change error type to string

[vgonkivs]

Overview

Change error type to string in order to marshal/unmarshal it correctly closes #66

Checklist

• [x] New and updated code has appropriate documentation
• [x] New and updated code has new and/or updated testing
• [x] Required CI checks are passing
• [x] Visual proof for any user facing features like CLI or documentation updates
• [x] Linked issues closed with keywords

[codecov-commenter]

Codecov Report

Merging #81 (7aa2164) into main (1ce71fd) will increase coverage by 0.02%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   65.94%   65.97%   +0.02%     
==========================================
  Files          35       35              
  Lines        2834     2836       +2     
==========================================
+ Hits         1869     1871       +2     
  Misses        811      811              
  Partials      154      154              

Impacted Files	Coverage Δ
sync/sync.go	63.00% <0.00%> (-1.33%)	:arrow_down:

... and 1 file with indirect coverage changes

[Wondertan]

@vgonkivs, could you address https://github.com/celestiaorg/go-header/issues/66#issuecomment-1629020766?

[vgonkivs]

made in a subsequent PR https://github.com/celestiaorg/go-header/pull/82