Sound system initialization should not be compulsory

[mbrethes]

The engine should keep running if sound card not present or sound system couldn't initialize, maybe with a warning that there will be no sound or something (see my pull request).

To reproduce:

• start mkxp on a system with no sound card or with a poorly configured sound configuration

Expected behavior:

• game runs with no sound (possibly with an alert about sound configuration)

Current behavior:

• the engine aborts execution before starting the game with the error message "Error opening OpenAL device"

Thanks :-)