Update message per review comment

Author: kevin-bates

notebook/services/contents/filemanager.py

@@ -607,5 +607,6 @@ def _validate_path(path):
 
         for char in invalid_chars:
             if char in path:
-                raise web.HTTPError(400, u"Path '{}' contains invalid characters {} relative to its platform.  "
-                                         u"Please edit the path and re-submit the request.".format(path, invalid_chars))
+                raise web.HTTPError(400, u"Path '{}' contains characters that are invalid for the filesystem. "
+                                         u"Path names on this filesystem cannot contain any of the following "
+                                         u"characters: {}".format(path, invalid_chars))