refactor(encoder): make errors private

[muktihari]

After careful consideration, I decide to make sentinel errors into private errors. I don't think we can use it to take a specific action with those errors, I think it's uncommon and maybe on special case only. For example, ErrInvalidWriter is actually not needed since user can just check the writer before passing to the encoder. Next case is ErrMissingDeveloperDataId, users are expected to write good fit's messages, having that error does not help users to take specific action since users still need to edit fit's messages before proceeding again.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (9f2ccc3) to head (f2fe5aa).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #539   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         4596      4596           
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  Hits          4596      4596           

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