When using bitbake -e in scripts, it would be helpful if the error
output appeared on stderr, not stdout. This change enables that building
upon the new bb.msg filters.
[YOCTO #5274]
(Bitbake rev: ebb797fc5c37d729e3cc8b2dc7156287d385c13b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>