audio: Set error message on dsp init failure.

if SDL_EnumUnixAudioDevices() fails to find any devices,
set an error message on the exit path. Without this,
SDL_Init() could fail without any message available
in SDL_GetError().

src/audio/dsp/SDL_dspaudio.c

@@ -308,6 +308,7 @@ DSP_Init(SDL_AudioDriverImpl * impl)
     InitTimeDevicesExist = SDL_FALSE;
     SDL_EnumUnixAudioDevices(0, look_for_devices_test);
     if (!InitTimeDevicesExist) {
+        SDL_SetError("dsp: No such audio device");
         return SDL_FALSE;  /* maybe try a different backend. */
     }