lib/oe/rootfs.py: warn the user if intercept hooks fail
The idea of postinstall intercept hooks is to run CPU intensive
postinstalls (like the ones generating font/pixbuf/icon caches) on host,
at rootfs generation time. So, ideally, the user would like to know if
the intercepts fail on host.
With this patch, the user will see warnings on console if intercept
hooks fail to execute.
