Send error message when CSRF token is missing

[kevinzg]

I was having the problem described in #13484. It turns out the 401 response was because the CSRF token was missing. This PR adds an error message for that case.

[codecov-io]

Codecov Report

Merging #13676 (4fe455b) into master (2791cc1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #13676   +/-   ##
=======================================
  Coverage   42.25%   42.25%           
=======================================
  Files         697      697           
  Lines       76596    76596           
=======================================
+ Hits        32363    32368    +5     
+ Misses      38919    38913    -6     
- Partials     5314     5315    +1     

Impacted Files	Coverage Δ
modules/context/api.go	70.00% <100.00%> (ø)
modules/queue/manager.go	62.13% <0.00%> (-2.96%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0.00%> (-1.54%)	⬇️
routers/api/v1/repo/pull.go	24.84% <0.00%> (-0.61%)	⬇️
modules/queue/workerpool.go	60.00% <0.00%> (+1.22%)	⬆️
services/pull/patch.go	55.68% <0.00%> (+1.70%)	⬆️
services/pull/check.go	51.82% <0.00%> (+2.91%)	⬆️
services/pull/temp_repo.go	29.78% <0.00%> (+3.19%)	⬆️
modules/charset/charset.go	73.03% <0.00%> (+4.49%)	⬆️

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