ImageMagick check uses convert, not identify

See #670 for details. This reverts a change from
a0c38a07a6. This caused problems on
Windows, but the fundamental problem is more general.

beets/util/artresizer.py

@@ -231,9 +231,10 @@ class ArtResizer(object):
 
 def get_im_version():
     """Return Image Magick version or None if it is unavailable
-    Try invoking ImageMagick's "convert"."""
+    Try invoking ImageMagick's "convert".
+    """
     try:
-        out = util.command_output([b'identify', b'--version'])
+        out = util.command_output([b'convert', b'--version'])
 
         if b'imagemagick' in out.lower():
             pattern = br".+ (\d+)\.(\d+)\.(\d+).*"
@@ -244,7 +245,8 @@ def get_im_version():
                         int(match.group(3)))
             return (0,)
 
-    except (subprocess.CalledProcessError, OSError):
+    except (subprocess.CalledProcessError, OSError) as exc:
+        log.debug(u'ImageMagick check `convert --version` failed: {}', exc)
         return None
 
 

test/test_embedart.py

@@ -35,7 +35,7 @@ def require_artresizer_compare(test):
 
     def wrapper(*args, **kwargs):
         if not ArtResizer.shared.can_compare:
-            raise unittest.SkipTest()
+            raise unittest.SkipTest("compare not available")
         else:
             return test(*args, **kwargs)