Don't allow sceUtility dialogs to overlap.

PSP doesn't allow this, and apparently some games depend on it
or else they start getting confused.
Savedata doesn't always resched, add some errcodes.
Add a basic impl of sceUtilityMsgDialogAbort().

Makes Enigmo work anyway.  Doesn't seem like a popular function.
Merge pull request #1119 from unknownbrackets/savedata

More sceUtility* changes