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 <art27@cantab.net>

Co-authored-by: Lauris BH <lauris@nix.lv>

routers/api/v1/repo/pull.go

@@ -759,8 +759,18 @@ func MergePullRequest(ctx *context.APIContext, form auth.MergePullRequestForm) {
 		return
 	}
 
-	if !pr.CanAutoMerge() || pr.HasMerged || pr.IsWorkInProgress() {
-		ctx.Status(http.StatusMethodNotAllowed)
+	if !pr.CanAutoMerge() {
+		ctx.Error(http.StatusMethodNotAllowed, "PR not in mergeable state", "Please try again later")
+		return
+	}
+
+	if pr.HasMerged {
+		ctx.Error(http.StatusMethodNotAllowed, "PR already merged", "")
+		return
+	}
+
+	if pr.IsWorkInProgress() {
+		ctx.Error(http.StatusMethodNotAllowed, "PR is a work in progress", "Work in progress PRs cannot be merged")
 		return
 	}
 
@@ -812,7 +822,7 @@ func MergePullRequest(ctx *context.APIContext, form auth.MergePullRequestForm) {
 
 	if err := pull_service.Merge(pr, ctx.User, ctx.Repo.GitRepo, models.MergeStyle(form.Do), message); err != nil {
 		if models.IsErrInvalidMergeStyle(err) {
-			ctx.Status(http.StatusMethodNotAllowed)
+			ctx.Error(http.StatusMethodNotAllowed, "Invalid merge style", fmt.Errorf("%s is not allowed an allowed merge style for this repository", models.MergeStyle(form.Do)))
 			return
 		} else if models.IsErrMergeConflicts(err) {
 			conflictError := err.(models.ErrMergeConflicts)