This improves the stacktraces dumped by bitbake when for example anonymous
python functions fail.
Also default to passing code strings to better_exec to match the behaviour of
simple_exec to aid the transition.
(Bitbake rev: 7e8205929ae953731a6854ea80b197847cff5771)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>