Use Err prefix for always return error implementation

[bogdandrutu]

Make this package consistent with a way more used package https://github.com/open-telemetry/opentelemetry-collector/blob/main/consumer/consumertest/err.go#L15

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.47%. Comparing base (78663f9) to head (bef56d1).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12874   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files         494      494           
  Lines       26993    26994    +1     
=======================================
+ Hits        24691    24692    +1     
  Misses       1818     1818           
  Partials      484      484           

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