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

[awrhygty@outlook.com]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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?

If error number is zero, warning message is not so important.
Messaging the warning in echo area will be enough.