Handle disk full more properly in sceIoWrite().
Return errors on disk full for file creation.
Return an error for new savedata on a full stick.
Show an on screen message when disk full is hit.
Show an error when screenshots fail to save.
Show an error when savedata can't be written.

Normally we'll log when the disk is full for files, but if we can't even
create the folder, we'll get errors trying to write into a non-existing
directory.
Merge pull request #7044 from unknownbrackets/memstick

Show and return errors on disk full