"error: couldn't mount because of unsupported optional features (240/244)"
This is more of a warning than an error. It occurs because the boot sequence attempts to read the boot partition with EXT3/EXT2, which is not the actual filesystem. It then continues by attempting ReiserFS/EXT4 and successfully locates the kernel. Since it gives the impression of an unclean installation, you can suppress these errors most easily by removing EXT3/EXT2 support in the kernel--neither filesystems will be used in MineOS. Otherwise, you can simply ignore it.