Report error if API merge is not allowed (#12528)
Report error if API merge is not allowed (#12528)

#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 <6a547fdd28db89120bdd70d4af1f9ba672ccd43a@cantab.net>

Co-authored-by: Lauris BH <f3041c4c693c07149804ea655e75ee8bb856bd26@nix.lv>
go-gitea/gitea,vdbt/gitea,sapk-fork/gitea,go-gitea/gitea,go-gitea/gitea,sapk-fork/gitea,cez81/gitea,cez81/gitea,sapk-fork/gitea,vdbt/gitea,cez81/gitea,sapk-fork/gitea