media: Capture stderr when running 'convert --version'

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