Provide helpful error messages when ImageMagick's convert isn't present

[voutilad]

Instead of a traceback when running tests like:

Traceback (most recent call last):
  File "/home/vagrant/seal-rookery/test.py", line 63, in test_can_force_regeneration_of_seals
    changed, skipped = convert_images.convert_images(forced=True)
  File "/home/vagrant/seal-rookery/seal_rookery/convert_images.py", line 84, in convert_images
    subprocess.Popen(command, shell=False).communicate()
  File "/usr/lib/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1343, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

Or a vague error when running update-seals -f like: [Errno 2] No such file or directory

There should be a more descriptive message suggesting that ImageMagick is not installed or on the current PATH.