Restructured Imagick to contain an instance of Imagick, rather than extending it

Fail gracefully when trying to start a new Imagick object. This is more consistent with GDBackend.

Added test for corrupt images.
All Imagick / GD tests appear to be passing.

Fixes #3441

@Zauberfisch @laminbarrow - does this also resolve any of the issues in #2781 ?