Report error if API merge is not allowed

[zeripath]

#12496 demonstrated that the API merge needs to return some information as
to why a merge has been disallowed with a status code 422.

This PR ensures that a reason is always returned.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12528 into master will decrease coverage by 0.02%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master   #12528      +/-   ##
==========================================
- Coverage   43.50%   43.48%   -0.03%     
==========================================
  Files         642      642              
  Lines       71003    71009       +6     
==========================================
- Hits        30892    30879      -13     
- Misses      35105    35117      +12     
- Partials     5006     5013       +7     

Impacted Files	Coverage Δ
routers/api/v1/repo/pull.go	25.05% <22.22%> (-0.31%)	⬇️
modules/indexer/stats/queue.go	52.94% <0.00%> (-23.53%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0.00%> (-1.54%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
services/pull/pull.go	41.57% <0.00%> (-0.47%)	⬇️

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[Tchoupinax]

LGTM

[6543]

🚀