Handle libmagic versions that don't support MAGIC_EXTENSION

extension support was added to libmagic in 2015 in
9190a18d09f25fb0ca6abe1fcbdba780f5077e45.  This change:

 - updates the test to handle various verions that return garbage /
   empty string.s
- throws an exception in magic.Magic if extension support is requested
  and we know its not supported.