media: Capture stderr when running 'convert --version'
At Wikimedia, $wgImageMagickConvertCommand points to a shell wrapper
that invokes ImageMagick convert with:
firejail --profile=/etc/firejail/mediawiki-converters.profile
firejail emits to stderr an information message:
Reading profile /etc/firejail/mediawiki-converters.profile
That ends up in HHVM stderr and is populated up to logstash as an error.
MediaWiki does check imagemagick version by running `convert -version`,
switch from wfShellExec() to wfShellExecWithStderr().
Bug: T158649
Change-Id: I78d1ef59533c605f59b42e10556bb595d6c2cc15