Avoid to print undefined image_size and partition_size

If dynamic partitioning is enabled and the partition size is not set,
we will get a KeyError before image_size or partition_size is calculated
when we try to catch exception in BuildImageMkfs.

Bug: 186704243
Test: build_image.py can correctly throw exception
Change-Id: I3d8c143ad5603d07fe94afb8bb911ead244f0bf7
Signed-off-by: Huang Jianan <huangjianan@oppo.com>