Have nicer 404 errors for covers

router.go

@@ -97,6 +97,7 @@ func (a *appData) buildRouter() http.Handler {
 
 			if err != nil {
 				narco.Error(ctx, w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+				return
 			}
 
 			rc, ext, err := a.cover.GetCover(AlbumID(ID))
@@ -104,10 +105,16 @@ func (a *appData) buildRouter() http.Handler {
 				defer closeOrPanic(rc, "Album cover "+name)
 			}
 			if err != nil {
+				if _, ok := err.(ErrorNotRegistered); ok == true {
+					narco.Error(ctx, w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+					return
+				}
 				narco.Error(ctx, w, err, http.StatusInternalServerError)
+				return
 			}
 			if ext != requestedExt {
 				narco.Error(ctx, w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+				return
 			}
 
 			io.Copy(w, rc)