Error codes

I've noticed that almost no HLE code even tries to handle and correctly report errors. This means that "innocent" errors such as missing SD files in blargSnes trigger assertion failures and crashes. In addition, a lot of code uses ObjectPool::GetFast which should really be using ObjectPool::Get instead. (Since GetFast does an out of bounds access if the handle is invalid.)

The intent of this effort is to make the system calls all deal with errors and return reasonable error codes, instead of crashing the emulator. (Or silently doing out of bounds access in non-debug builds in some cases...)

This is in no way mergeable and is merely an RFC to get feedback on the coding style and solutions to use.
The returned handle was never used in any of the call sites, and this is a Citra-internal function, so I just removed it.