Improve access to Imagemagick "identify" and "convert"

Imagemagick looks for the "identify" binary at /bin/identify, where it might not necessarily be. Similarly, "convert" could be somewhere unexpected. We should add them to the admin form, and improve error reporting when these binaries can't be found.

Diego Pino Navarro asked me to create this ticket; he'll have more details on the issue and its solution.

Relevant discussion: https://groups.google.com/d/topic/islandora/r5157QAXDIw/discussion