add stereo track combining error to errors list

[LWinterberg]

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 audacity/audacity#4737 (comment)

[SteveDaulton]

Wouldn't it be better for Audacity 3.4 to offer to fix the problem (resample the lower sample rate channel to match the other channel) rather than just telling the user that their project is no longer supported?

[LWinterberg]

Yes, but we aren't sure if even a single user has this issue. If this error message gets meaningful traction in sentry it can be patched in later.