fix: print error messages to stderr

[ahaoboy]

Using the following command will cause a warning message to be written to the png file

cat ./test.svg | resvg - -c > test.png

test.png

Warning: Make sure to set --resources-dir when reading SVG from stdin.
�PNG


Removed the colon for consistency in error messages.

Warning Make sure to set --resources-dir when reading SVG from stdin.
Warning (in fontdb:241): Failed to load a font face 0 from 'C:\Windows\Fonts\mstmc.ttf' cause malformed font.
Warning (in usvg::text::layout:1551): Fallback from Times New Roman to Cascadia Code.
Warning (in usvg::text::layout:1551): Fallback from Courier New to Cascadia Code.
Warning (in usvg::text::layout:1551): Fallback from Times New Roman to Cascadia Code.

[RazrFalcon]

Make sense. Thanks. But can you keep colons (:)?

[ahaoboy]

But can you keep colons (:)?

done

[RazrFalcon]

Thanks!