bug#63231: 28.1; image-converter should remove warning messages while conversion

[Eli Zaretskii]

> From: awrhygty@outlook.com
> Date: Tue, 02 May 2023 17:02:21 +0900
> 
> 
> I have a WEBP image with a GRAY ICC profile which can be converted into
> PNG format successfully by ImageMagick command,
> but with a warning message such like:
> 
>   magick convert test.webp test.png
>   convert: profile 'icc': 'GRAY': Gray color space not permitted on RGB PNG 
> `test.png' @ warning/png.c/MagickPNGWarningHandler/1748.
> 
> When trying to display the image within a emacs buffer,
> image-converter takes both standard output and standard error while
> converting it into a PNG data, and emacs fails to display the image.
> 
> A simple avoiding way is adding -quiet option to converting command.
> Ideally, image-converter should split standard error into another buffer.

What if the errors are fatal? don't we want to present the user with
the error messages?