fix the flickering by delaying and throwing

Explanation coming on my blog, math2001.github.io, something like
"lessons learned from re writing MarkdownLivePreview"

live-testing/images.md

@@ -1,5 +1,7 @@
 I'm not sure that it **actually** going to work, but it seems nicer than the [previous version][prev]
 
+this is a test, hello world
+
 This is the first image from the local file system (absolute path, sorry, it's not going
 to work on your system unless your username is math2001):
 

live-testing/test.md

@@ -1,4 +1,4 @@
-# hello world  
+# hello world
 
 This is a *test*. Some inline `[2]code()`.
 
@@ -13,6 +13,4 @@ if input("answer yes") != 'yes':
 
 this flickering is really annoying...
 
-this
-
-It looks like it's gone... Oh wait nah, it's still here...
+It looks like it's gone... Oh wait nah, it's still here...