Reduce dual pack + channel from error to warn

[JohnnyMarnell]

Hello and thanks for the great work here! I tried to render two Atmos BWF's, one created with Dolby Atmos Renderer and another with Fiedler Atmos Composer, both write axml with stream formats referencing both pack + channel formats.

Changing this to a warning instead allows the process to succeed and sounds correct / expected loudness.