insane: Promote warnings into errors

insane: Promote warnings into errors

There are several warnings the build can emit which show real problems
in the way recipes are being built. Some distros like poky and the Yocto
Project autobuilder have been fixing the issues these show for some time.

OE-Core has therefore been clean of these errors and warnings for some time.
Promote warnings into errors to match the defaults in poky to encourage
people to fix these real issue and reduce confusion when people switch
distros or customise.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>