No error feedback as convert returns 0 even with error messages on stderr

I was doing some testing (http://drupal.org/node/818798#comment-3927766) and found that the return code from proc_close is 0, regardless any (error) messages send to stderr.

file: imageapi_imagemagick.module
function: _imageapi_imagemagick_exec

command line:

"C:\Program Files\ImageMagick-6.6.1-Q16\convert.exe" "D:\Projecten\Drupal7\www\modules\image\sample.png" -resize 180x135! -crop 180x135+0+0!   -background '#333333'   -crop   '184x139-2-2!'   -extent '184x139-2-2!'  -quality "75" "D:\Projecten\Drupal7\sample.png"

Output on stderr (as saved in $errors):

convert.exe: unrecognized color `'#333333'' @ warning/color.c/GetColorInfo/946.
convert.exe: invalid geometry `'184x139-2-2!'' @ error/geometry.c/ParseGravityGeometry/990.
convert.exe: unrecognized color `'#333333'' @ warning/color.c/GetColorInfo/946.
convert.exe: invalid geometry `'184x139-2-2!'' @ error/geometry.c/ParseGravityGeometry/990.
convert.exe: unrecognized color `'#333333'' @ warning/color.c/GetColorInfo/946.
convert.exe: invalid geometry `'184x139-2-2!'' @ error/geometry.c/ParseGravityGeometry/990.
convert.exe: unrecognized color `'#333333'' @ warning/color.c/GetColorInfo/946.
convert.exe: invalid geometry `'184x139-2-2!'' @ error/geometry.c/ParseGravityGeometry/990.

Configuration:

Windows Vista
ImageMagick 6.6.1-6 2010-04-23 Q16

So I think it is better to check for return code OR $errors in deciding whether or not to call trigger_error() (and inform the user). But this might lead to false positives as I'm not sure about when convert sends output to stderr while returning 0 at the same time.