Remove nil error from being printed in "persist metadata" error message

[malancas]

When go-tuf is unable to persist the TUF metadata, I receive the error "failed to persist metadata: %!w(<nil>)". Because the code already checks if the error is not nil and returns it if not on line 651, the "failed to persist" error message will always try to print a nil error.

[MDr164]

I'm currently preparing a really big PR that takes care of error checking in general to avoid things like this from happening but good thing you spotted this one here early.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.94%. Comparing base (14cf073) to head (e2b2ca7).
Report is 20 commits behind head on master.

Files	Patch %	Lines
metadata/updater/updater.go	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
+ Coverage   70.51%   72.94%   +2.43%     
==========================================
  Files          10       10              
  Lines        2123     1656     -467     
==========================================
- Hits         1497     1208     -289     
+ Misses        505      327     -178     
  Partials      121      121              

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