Adding better AVB error message

Also removing the requirement of BOARD_BOOTIMAGE_PARTITION_SIZE when
BOARD_AVB_ENABLE is set. Some targets (e.g., emulator) doesn't build
boot.img when AVB is enabled.

Bug: 112293933
Test: build a failure case and checks the error output is expected
Change-Id: I290c707719193ddaedcd9fd0b31de566ad17078c