Right now we do not display anything if we have errors/warnings that effect the return code of bitbake. We should try to explain why the return code was not 0 when exiting.
Fixed in master: http://git.yoctoproject.org/cgit.cgi/poky/commit/?id=e4a365098a9550facae9af069d4b58770dabfd91
The feature is tested workable against 1.2 M2.