Improve 404 error message

[guillep2k]

Since 404 does not exactly mean "page not found" as it should, I thought it would be nice to add some clarification about the error message for better usability.

[codecov-io]

Codecov Report

Merging #10214 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10214      +/-   ##
==========================================
- Coverage   43.64%   43.63%   -0.02%     
==========================================
  Files         576      576              
  Lines       79716    79716              
==========================================
- Hits        34791    34783       -8     
- Misses      40627    40635       +8     
  Partials     4298     4298

Impacted Files	Coverage Δ
services/pull/temp_repo.go	29.05% <0%> (-2.57%)	⬇️
services/pull/patch.go	60.37% <0%> (-2.52%)	⬇️
services/pull/check.go	32.92% <0%> (-2.44%)	⬇️
modules/queue/unique_queue_disk_channel.go	54.48% <0%> (+1.92%)	⬆️

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