follower: update follower node error messages.

[winder]

Summary

The feature was renamed from sync to follower and this error message was not updated along with it.

There is a possibility that user code could be checking for this message, so this could be considered a breaking change. This error message also indicates a configuration issue, so error handling would be unable to resolve it.

My recommendation is to make the breaking change.

Test Plan

[codecov]

Codecov Report

Merging #5797 (1cae0a7) into master (d4b4086) will increase coverage by 0.23%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #5797      +/-   ##
==========================================
+ Coverage   55.32%   55.56%   +0.23%     
==========================================
  Files         475      475              
  Lines       66850    66850              
==========================================
+ Hits        36988    37142     +154     
+ Misses      27341    27187     -154     
  Partials     2521     2521              

Files	Coverage Δ
node/follower_node.go	45.16% <66.66%> (+3.22%)	⬆️

... and 27 files with indirect coverage changes

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[jasonpaulos]

I think the added benefit of clarity here outweigh the potential risks