Return sample message for login error in api context

[a1012112796]

As title, will return a sample message instead of full html page.

[codecov-commenter]

Codecov Report

Merging #12994 into master will increase coverage by 0.03%.
The diff coverage is 10.52%.

@@            Coverage Diff             @@
##           master   #12994      +/-   ##
==========================================
+ Coverage   42.51%   42.54%   +0.03%     
==========================================
  Files         672      672              
  Lines       73699    73715      +16     
==========================================
+ Hits        31336    31365      +29     
+ Misses      37284    37265      -19     
- Partials     5079     5085       +6     

Impacted Files	Coverage Δ
modules/context/auth.go	18.47% <10.52%> (-2.58%)	⬇️
models/repo_indexer.go	81.66% <0.00%> (-3.34%)	⬇️
models/repo_language_stats.go	61.73% <0.00%> (-1.74%)	⬇️
modules/indexer/stats/queue.go	64.70% <0.00%> (ø)
services/pull/pull.go	41.27% <0.00%> (+0.49%)	⬆️
models/pull.go	55.17% <0.00%> (+0.62%)	⬆️
modules/queue/unique_queue_channel.go	64.51% <0.00%> (+6.45%)	⬆️
services/pull/check.go	47.69% <0.00%> (+16.15%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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[zeripath]

I think these suggestions read a bit better but otherwise LGTM.