On merge of already closed PR redirect back to the pulls page (#10010)

* On merge of already closed PR redirect back to the pulls page

* More redirects
* As per @6543

Co-Authored-By: 6543 <6543@obermui.de>

routers/repo/pull.go

@@ -679,7 +679,13 @@ func MergePullRequest(ctx *context.Context, form auth.MergePullRequestForm) {
 		return
 	}
 	if issue.IsClosed {
-		ctx.NotFound("MergePullRequest", nil)
+		if issue.IsPull {
+			ctx.Flash.Error(ctx.Tr("repo.pulls.is_closed"))
+			ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(issue.Index))
+			return
+		}
+		ctx.Flash.Error(ctx.Tr("repo.issues.closed_title"))
+		ctx.Redirect(ctx.Repo.RepoLink + "/issues/" + com.ToStr(issue.Index))
 		return
 	}
 
@@ -691,12 +697,20 @@ func MergePullRequest(ctx *context.Context, form auth.MergePullRequestForm) {
 		return
 	}
 	if !allowedMerge {
-		ctx.NotFound("MergePullRequest", nil)
+		ctx.Flash.Error(ctx.Tr("repo.pulls.update_not_allowed"))
+		ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(issue.Index))
 		return
 	}
 
-	if !pr.CanAutoMerge() || pr.HasMerged {
-		ctx.NotFound("MergePullRequest", nil)
+	if !pr.CanAutoMerge() {
+		ctx.Flash.Error(ctx.Tr("repo.pulls.no_merge_not_ready"))
+		ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(issue.Index))
+		return
+	}
+
+	if pr.HasMerged {
+		ctx.Flash.Error(ctx.Tr("repo.pulls.has_merged"))
+		ctx.Redirect(ctx.Repo.RepoLink + "/pulls/" + com.ToStr(issue.Index))
 		return
 	}