add stereo track combining error to errors list

> I tested this on W10 with the latest master: audacity-win-3.4.0-alpha-20230609+0ec7ab2-x64-msvc2022
>
> This works fine now:
>
> 1. The creation of a stereo track is blocked
> a) if one of the tracks has a Rate change from the TCP
> b) if one of the tracks is Resampled with Tracks > Resample
> 2. Projects created in earlier Audacity versions with stereo tracks with mismatched channels (created either way) cannot be opened on 3.4.0 alpha.
>
> @LWinterberg **_BUT_** the error message that is shown has a help link to the GitBook Audacity Support errors page, but there is nothing on that page to advise users as to how they can correct this situation.
>
> 
>
> Links to this page: https://support.audacityteam.org/troubleshooting/error-codes
>
> And note that the error message in the image above does not actually have an "error code" as such _(but then several other errors dealt with on that page do not have errors codes either)_.

via @petersampsonaudacity in https://github.com/audacity/audacity/issues/4737#issuecomment-1585589077