You can not mount a hibernated partition (At least in a Read/Write state). The reason behind this is that if you happen to copy something from your Ubuntu system to the Window's hibernated system, it will be lost when you boot Windows.
This is actually a safety feature put by the mounting process in Ubuntu because many users complaint that when they copied something from Ubuntu to Windows, they lost the data. It later came to light, that the Windows system was hibernated, creating this problem because in hibernation, everything is frozen (The system is read only) and is not changed until you boot again from within Windows.
The recommendation as mentioned by the warning is to boot Windows 8 and shut the system correctly. not by hibernation, but by the shutdown option.
If you still want to mount the unit, it should be in a READ ONLY state. For this we could do the following:
sudo mount -o ro /dev/sda2 /mnt
Where ro is Read Only, /dev/sda2 is the partition you want to mount and /mnt is where you want to mount this partition.
Still I would go with going to Windows 8, shutting down the computer (No hibernation) and going back to Ubuntu and enjoy a warm and sweet moment with a Read/Write partition.
UPDATE: If your Windows 8 system always stays in a hibernated state, please read the section about TROUBLESHOOT in Installing Ubuntu Alongside a Pre-Installed Windows with UEFI . It covers the
weird behavior "feature" Windows 8 offers to users automatically to make the system more confusing "better" for them.