Handle BLKDISCARD failures

In the block updater, if BLKDISCARD fails, the error is silently
ignored and some of the blocks may not be erased. This means the
target partition will have inconsistent contents.

If the ioctl fails, return an error and abort the update.

Bug: 20614277
Change-Id: I33867ba9337c514de8ffae59f28584b285324067