preview: support skipping over single missing images inside large albums

preview.go

@@ -109,10 +109,29 @@ html, body {
 		m, err := this.Metadata(fileID)
 		if err != nil {
 			if os.IsNotExist(err) {
+
+				// If this is just one image out of many, show a 404 box and continue to show the other entries
+				// But if this is only a single image requested, abandon the whole pageload
+
+				if len(fileIDs) == 1 {
 					http.Error(w, "Not found", 404)
 					return
 				}
 
+				tmpl += `
+					<div class="entry">			
+						<div class="thumbnail">
+							<img loading="lazy" src="/nothumb_340.png"></a>
+						</div>
+						<div class="properties">
+							Requested ID ` + html.EscapeString(fileID) + ` not found in storage (404)
+						</div>
+					</div>
+				`
+
+				continue
+			}
+
 			log.Println(err.Error())
 			http.Error(w, "Internal error", 500)
 			return