Fix DefaultHTTPErrorHandler with enabled debug (#1477)

[lammel]

This PR will correct the misbehaviour described in issue #1477.

With enabled debug still the correct error will be returned and not overwritten.
Tests are added, where the now expected errors are shown

[codecov]

Codecov Report

Merging #1666 into master will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1666      +/-   ##
==========================================
+ Coverage   83.81%   83.97%   +0.16%     
==========================================
  Files          28       28              
  Lines        1909     1910       +1     
==========================================
+ Hits         1600     1604       +4     
+ Misses        195      194       -1     
+ Partials      114      112       -2     

Impacted Files	Coverage Δ
echo.go	84.75% <100.00%> (+1.17%)	⬆️

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[lammel]

Thanks for the review @pafuent .
As only Debug mode is affected, we'll make it happen.