[BUG] Reflect Cargo index state in settings

- Currently in the Cargo section of the packages setting menu two
buttons are always shown, "Initalize index" and "Rebuild index", however
only of these should be shown depending on the state of the index, if
there's no index the "Initalize index" button should be shown and if
there's an index the "Rebuild index" button should be shown. This patch
does exactly that.
- Resolves #2628

routers/web/shared/packages/packages.go

@@ -4,16 +4,19 @@
 package packages
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
 
 	"code.gitea.io/gitea/models/db"
 	packages_model "code.gitea.io/gitea/models/packages"
+	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/services/context"
 	"code.gitea.io/gitea/services/forms"
@@ -29,6 +32,12 @@ func SetPackagesContext(ctx *context.Context, owner *user_model.User) {
 	}
 
 	ctx.Data["CleanupRules"] = pcrs
+
+	ctx.Data["CargoIndexExists"], err = repo_model.IsRepositoryModelExist(ctx, owner, cargo_service.IndexRepositoryName)
+	if err != nil {
+		ctx.ServerError("IsRepositoryModelExist", err)
+		return
+	}
 }
 
 func SetRuleAddContext(ctx *context.Context) {
@@ -240,7 +249,11 @@ func RebuildCargoIndex(ctx *context.Context, owner *user_model.User) {
 	err := cargo_service.RebuildIndex(ctx, owner, owner)
 	if err != nil {
 		log.Error("RebuildIndex failed: %v", err)
-		ctx.Flash.Error(ctx.Tr("packages.owner.settings.cargo.rebuild.error", err))
+		if errors.Is(err, util.ErrNotExist) {
+			ctx.Flash.Error(ctx.Tr("packages.owner.settings.cargo.rebuild.no_index"))
+		} else {
+			ctx.Flash.Error(ctx.Tr("packages.owner.settings.cargo.rebuild.error", err))
+		}
 	} else {
 		ctx.Flash.Success(ctx.Tr("packages.owner.settings.cargo.rebuild.success"))
 	}