all: fix some errcheck warnings

Created by: keegancsmith

A new meditation for yuppie software engineers. Run errcheck on our codebase and fix some of the warnings. 5 minutes a day will lead to a better, happier and examined life.

Most fixes are just explicitly marking we are ignoring the error. This is the case often since we are writing a response and there is no reasonable recourse if it fails. Some changes handle the error.

Created by: codecov[bot]

Codecov Report

Merging #7800 into master will not change coverage. The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master    #7800   +/-   ##
=======================================
  Coverage   40.58%   40.58%           
=======================================
  Files        1262     1262           
  Lines       66211    66211           
  Branches     6182     6182           
=======================================
  Hits        26871    26871           
  Misses      36877    36877           
  Partials     2463     2463

Impacted Files	Coverage Δ
cmd/symbols/internal/pkg/ctags/parser.go	0% <0%> (ø)
cmd/github-proxy/github-proxy.go	37.87% <0%> (ø)
cmd/frontend/internal/auth/userpasswd/handlers.go	0% <0%> (ø)
cmd/frontend/internal/app/app.go	0% <0%> (ø)
cmd/frontend/internal/app/sign_out.go	0% <0%> (ø)
internal/store/zipcache.go	46.23% <100%> (ø)
cmd/frontend/internal/app/misc_handlers.go	38.88% <100%> (ø)
enterprise/cmd/frontend/auth/saml/middleware.go	49.58% <100%> (ø)
cmd/gitserver/server/server.go	46.76% <66.66%> (ø)