Chain Imagick exceptions instead of hiding.

If Imagick throws while reading the file, the original exception
is hidden and a new one is thrown instead. This PR passes the
original exception along in the $previous parameter, so you can
figure out what exactly went wrong under the hood 😎

src/Intervention/Image/Imagick/Decoder.php

@@ -23,7 +23,9 @@ class Decoder extends \Intervention\Image\AbstractDecoder
 
         } catch (\ImagickException $e) {
             throw new \Intervention\Image\Exception\NotReadableException(
-                "Unable to read image from path ({$path})."
+                "Unable to read image from path ({$path}).",
+                0,
+                $e
             );
         }
 
@@ -81,7 +83,9 @@ class Decoder extends \Intervention\Image\AbstractDecoder
 
         } catch (\ImagickException $e) {
             throw new \Intervention\Image\Exception\NotReadableException(
-                "Unable to read image from binary data."
+                "Unable to read image from binary data.",
+                0,
+                $e
             );
         }