[API] Error dont return html

[6543]

regression of #11355

Explanation:
ctx.ServerError() was not overloaded by APIContext so it would respond with an html error page instead of json :/

in the past I rewrote it with ctx.Error() or ctx.InternalServerError() but this issue is so common that it's better to overload this function instead

[6543]

@a1012112796 ping ;)

[codecov-io]

Codecov Report

Merging #14440 (8dc76bd) into master (525d75c) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #14440      +/-   ##
==========================================
- Coverage   41.76%   41.76%   -0.01%     
==========================================
  Files         748      748              
  Lines       80103    80105       +2     
==========================================
- Hits        33455    33454       -1     
- Misses      41125    41130       +5     
+ Partials     5523     5521       -2     

Impacted Files	Coverage Δ
modules/context/api.go	69.07% <0.00%> (-0.93%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
routers/repo/view.go	40.95% <0.00%> (-0.63%)	⬇️
models/gpg_key.go	53.33% <0.00%> (-0.58%)	⬇️
services/pull/pull.go	42.15% <0.00%> (-0.50%)	⬇️
modules/process/manager.go	75.00% <0.00%> (+2.50%)	⬆️
modules/charset/charset.go	73.03% <0.00%> (+4.49%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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