[llvm][TableGen][Jupyter] Show llvm-tblgen not found error in notebook

Previously this message was only shown on the command line,
which is not much help if you can't see that.
(you've full screened the browser or you aren't running Jupyter
on the same machine)

Instead return the error as stderr which will get printed in
the notebook just like stderr from llvm-tblgen would.

I've refactored the message sending along the way. Note that
even when we do not send a stream, we still need to send the
status reply. The send_... methods will do that for you.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D142531