mach: gamemode initialization errors should be silent if gamemode not installed

[slimsag]

See #458

Currently if you do not have gamemode installed (many will not), it screams:

error: Gamemode error error.RequestFailed -> dlopen failed - libgamemode.so: cannot open shared object file: No such file or directory

Maybe we can string-contains check the error message to see if it is just complaining the .so is missing, in which case ignore the error silently - otherwise log it.