Obtain an exclusive lock on the active save file

[StephenCWills]

As AJenbo indicated in #7674 (comment), it would be nice if the game client could capture an exclusive lock on the active save file when opening a game session. This should allow us to completely eliminate errors that might occur during runtime caused by other processes on the system, such as cloud synchronization or virus scanning, when we are attempting to save the player's progress.

There are some complications involved that will need to be considered. In particular, we'll need to ensure that we are still able to run multiple clients on the same system without any major issues. The following list seems to me to be a reasonable set of requirements to ensure that multiclienting is not adversely affected.

• The game should refrain from getting an exclusive lock on any save files before the first loading screen. Furthermore, the client should only keep an exclusive lock on the active character's save file, as well as the stash file, until the game session is ended and the game client returns to the main menu.
• If the client is unable to open a save file on the character select screen, that save file should be skipped and the character should not appear in the list. The game should definitely not crash in this case.
• If the client is unable to open the save file the player selected when creating a game session, this would be an appropriate time to display an error message and crash the game client. That said, it would obviously be a bit better if we were able to simply abort the loading process and return to the menus instead of crashing.
• If we are unable to open the stash file when creating a new game session, we should disable the stash entirely for that game session. We will also need to inform the player that the stash is unavailable so they're not caught off-guard in the middle of their play session.