Add error handling for HTTP response codes other than 200.

This works also for local errors such as no connection as browsers
return a response code of 0 in that case

templates/pad.tmpl

@@ -434,7 +434,11 @@
             xhr.send(formData);
             xhr.onreadystatechange = function(){
                 if(xhr.readyState == 4){
+                    if (xhr.status == 200){
                         processResponse(xhr.responseText, file.identifier);
+                    } else {
+                        processFailure(file.identifier);
+                    }
                 }
             }
         }
@@ -445,6 +449,10 @@
             textarea.value = textarea.value.replace(getMarkdownRegex(identifier), "["+ description +"]("+response.url+")");
         }
 
+		function processFailure(identifier){
+            textarea.value = textarea.value.replace(getMarkdownRegex(identifier), "");
+        }
+
         function getMarkdownRegex(identifier){
             return new RegExp('\\[' + identifier + '\\]\\(.*?\\)'); 
         }