create and save err if new image file is not successfully created

[MarkZsombor]

What does this do?

Deals with Issue #65

Currently create/2 and save/2 fail silently if the file is not created as they don't check the result of the System.cmd call.

System.cmd returns a tuple with the result and the command exit status, so by pattern matching the result of cmd_mogrify and cmd_convert against a successful attempt ({_, 0}) these functions will now raise an error for unsuccessful exist statuses.

This is similar to the implementation for buffer in create/2.

How to test?

Try to save a file with an illegal file type, and error should be raised.

iex(3)> Mogrify.open("sample.png") |> format("jpg") |> save
%Mogrify.Image{
  animated: false,
  buffer: nil,
  dirty: %{},
  ext: ".jpg",
  format: "jpg",
  frame_count: 1,
  height: nil,
  operations: [],
  path: "/var/folders/hs/6c8zskxs7891h185psnyk3wm0000gn/T/18777-sample.jpg",
  width: nil
}
iex(4)> Mogrify.open("sample.png") |> format("doggos") |> save
** (MatchError) no match of right hand side value: {"mogrify: unable to open image 'doggos:': No such file or directory @ error/blob.c/OpenBlob/3537.\nmogrify: unrecognized image format `doggos' @ error/mogrify.c/MogrifyImageCommand/5062.\n", 1}
    (mogrify 0.7.4) lib/mogrify.ex:30: Mogrify.save/2

What needs to be considered ?

It may be nice to have it throw a better error, but I'd leave the shape of that open for discussion

[talklittle]

Thanks. Agree that better error would be nice as a future improvement. Any error is better than silent failure so merging now.