Ignore 404 errors for release group json

We currently have no easy mean to know if there is cover art for the release group as we do for the release.

Not a perfect solution, ideas and patches are welcome.
We should not query the json file at all if we know there is no cover art for this release group.

picard/coverartproviders/caa.py

@@ -42,6 +42,9 @@ class CoverArtProviderCaa(CoverArtProvider):
 
     NAME = "Cover Art Archive"
 
+    ignore_json_not_found_error = False
+
+
     def __init__(self, coverart):
         CoverArtProvider.__init__(self, coverart)
         self.caa_types = map(unicode.lower, config.setting["caa_image_types"])
@@ -125,7 +128,8 @@ class CoverArtProviderCaa(CoverArtProvider):
         """Parse CAA JSON file and queue CAA cover art images for download"""
         self.album._requests -= 1
         if error:
-            self.error(u'CAA JSON error: %s' % (unicode(http.errorString())))
+            if not (error == 203 and self.ignore_json_not_found_error):
+                self.error(u'CAA JSON error: %s' % (unicode(http.errorString())))
         else:
             try:
                 caa_data = json.loads(data)

picard/coverartproviders/caa_release_group.py

@@ -31,6 +31,9 @@ class CoverArtProviderCaaReleaseGroup(CoverArtProviderCaa):
 
     NAME = "CaaReleaseGroup"
 
+    ignore_json_not_found_error = True
+
+
     def enabled(self):
         if not config.setting['ca_provider_use_caa']:
             log.debug("Cover art from Cover Art Archive disabled by user")