fix(gateway): do not act on template errors

[hacdias]

Fixes #487. When we try to execute a template, we have already written the header. In some places, we were already ignoring the template execution error. In others, we were calling webError. That would lead to a duplicate header writing warning.

There is not much that can be done if a template fails to execute: we could perhaps log it, but so far what we've done in other places is to ignore. A way to circumvent this would be to write to a buffer and then write to http.ResponseWriter. That would allow us to catch the template error, but writing can always fail (e.g., broken connection).

I wouldn't care much about this and I think just ignoring the template writing error is probably fine. If errors happen, they will most likely be writing errors, so broken connections and other network issues, most likely.

[codecov]

Codecov Report

Merging #515 (5bfcf4f) into main (fe55533) will decrease coverage by 0.07%.
The diff coverage is 35.71%.

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
- Coverage   65.58%   65.52%   -0.07%     
==========================================
  Files         207      207              
  Lines       25540    25543       +3     
==========================================
- Hits        16750    16736      -14     
- Misses       7323     7335      +12     
- Partials     1467     1472       +5     

Files	Coverage Δ
gateway/handler_unixfs_dir.go	67.27% <100.00%> (ø)
gateway/handler_codec.go	62.85% <60.00%> (+0.77%)	⬆️
gateway/errors.go	83.33% <25.00%> (-2.14%)	⬇️
gateway/handler.go	76.54% <0.00%> (-0.14%)	⬇️

... and 7 files with indirect coverage changes

[lidel]

Thanks! Lgtm.