buildstats: check IMAGE_ROOTFS exists before checking its size

buildstats: check IMAGE_ROOTFS exists before checking its size

After 0d6b7276003f1afabc6de683f663540327d52bdc, the exceptions are
correctly checked if the rootfs size check fails. In case of a
failure a build error is triggered.

However, there are cases where this is known to fail (e.g.,
with meta-swupd the rootfs for swupd images is other than IMAGE_ROOTFS).

Because of that, check IMAGE_ROOTFS exists before trying to get the
size of it. Also, in case of any error catched as err, simply print
out a warning.

(From OE-Core rev: b4929542ff01a24bea5edd1c40e3174f55e213ff)

Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>