don't raise 404 in get_absolute_path

raise in validate_absolute_path, where it belongs

IPython/html/base/handlers.py

@@ -330,13 +330,16 @@ class FileFindHandler(web.StaticFileHandler):
                 abspath = os.path.abspath(filefind(path, roots))
             except IOError:
                 # IOError means not found
-                raise web.HTTPError(404)
+                return ''
             
             cls._static_paths[path] = abspath
             return abspath
     
     def validate_absolute_path(self, root, absolute_path):
         """check if the file should be served (raises 404, 403, etc.)"""
+        if absolute_path == '':
+            raise web.HTTPError(404)
+        
         for root in self.root:
             if (absolute_path + os.sep).startswith(root):
                 break