fastboot/sparse: propagate error code

sparse_file_write_block calls functions that might failed.  This patch
makes sparse_file_write_block catch the error code and propagate it.

Without this patch, fastboot crashes on a segmentation fault if
usb_write() fail during a sparse file image flashing.

Change-Id: If9c0e8dfac8fa380628a2504e13abe8cf7d62726
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>