Add assertions for graphics command buffer allocation

When memory for a command or data in the command buffer cannot be allocated in `CGraphics_Threaded::AddCmd` and `CGraphics_Threaded::AllocCommandBufferData` the command buffer is cleared so it should always be possible to allocated memory successfully on the second try. Therefore assertions are added and the return values and inconsistent checks of the functions are removed.

The usage of `AddCmd` is simplified by automatically deriving an error message based on the type of the template parameter.