Add error for fork already existing

[jolheiser]

Partial fix for #7154 (considered partial because it addresses the error being misleading, however I think the issue author was asking for something else and this was just a byproduct)

The error returns the authed username, the repo they are trying to fork, and the name of the already existing fork (in case it was renamed)

[lunny]

Except CI failed.

[codecov-io]

Codecov Report

Merging #7185 into master will decrease coverage by 0.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #7185      +/-   ##
==========================================
- Coverage   41.53%   41.52%   -0.02%     
==========================================
  Files         449      449              
  Lines       61302    61308       +6     
==========================================
- Hits        25462    25458       -4     
- Misses      32488    32495       +7     
- Partials     3352     3355       +3

Impacted Files	Coverage Δ
models/repo.go	48.27% <100%> (+0.02%)	⬆️
models/error.go	32.39% <60%> (+0.3%)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️
models/gpg_key.go	55.83% <0%> (-0.84%)	⬇️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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