ExtractOIIOTranscode: Log warning instead of raise error if media can't be transcoded due to unknown RGBA channels

[BigRoy]

Changelog Description

Allow ExtractOIIOTranscode to pass with a warning if it can't find the RGBA channels in source media instead of raising error

Additional info

Over time we've had annoying reports where studios were unable to publish their renders because the publishing system (due to user configuration) tries to transcode e.g. a Z-depth AOV layer which may only have a named Z or Y channel and has no R, G, B or A channels and hence we don't really know what layer to process.

Instead of this we could also implement detecting more cases, so that if RGBA is not found in the file that maybe we allow to fall back to channels:

• XYZ
• Y
• Z (usually for z-depth?)

Here's metadata output for a file that fails without this PR:

<nchannels>1</nchannels>
<channelnames>
<channelname>Y</channelname>
</channelnames>
<alpha_channel>-1</alpha_channel>
<z_channel>-1</z_channel>
<deep>0</deep>

It's an Arnold Z-depth render AOV.

Testing notes:

1. Unknown RGBA channels will now log a warning instead of disallow publishing altogether.

[jakubjezek001]

I wonder if we always want to make the process fail gracefully if RGB channels are missing or there might be cases where this is not desired and the process should consider it hard fail. In that case we should probably have some switch for the detection. But to be frank I am not currently aware of the case we would need to have it hard fail all the time.

[iLLiCiTiT]

I wonder how it ends up being marked for review if it cannot be used for review?