The command line checks for command line utilities are done by trying to
access the executables and by throwing an exception when not found. This
lead to:
- The mat2 cmd line --check-dependencies option failing.
- The ffmpeg unit tests failing when ffmpeg isn't installed (even though
  it's an optional dependency).

This patch fixes it.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>