[This is a spin-off of bug 346450 comment 14 , 15 , 16] Currently many users of JS_SetPrivate check for possible failure of JS_SetPrivate. But this function is infallible so such checks are unnecessary. We should remove such checks and either fix JS_SetPrivate so it would return void or at least document that the function always returns true if backward compatibility issues can not be addressed with preprocessor hacks or something.

Already done.