No clear indication of the bookmark edit being rejected in bukuserver

[LeXofLeviafan]

When buku rejects an update of existing bookmark (i.e. when the URL is being changed to one already associated with another bookmark), the edit form page simply refreshes with no error message indicating an update failure (the form still contains values entered by the user, but they haven't been submitted despite the lack of any visible errors). If the edit was done in a modal form, the edit page is opened – again, with no clear indication of the edit being rejected.