* lisp/ob-exp.el (org-babel-exp-do-export): Display warning when
:exports value is not known. Document nil return value.
(org-babel-exp-process-buffer): Do not remove code block when
`org-babel-exp-do-export' returns nil.
Reported-by: Max Nikulin <manikulin@gmail.com>
Link: https://orgmode.org/list/usc9jn$g2r$1@ciao.gmane.io