Improve handling of unmatched releases

Errors encountered when a matching release cannot be found for the disc
are now handled more appropriately. This provides a better indication to
the user of this problem.

ripcd.py

@@ -239,7 +239,6 @@ def get_release_by_id(mbid):
             'labels',
         ],
     )
-    import pdb; pdb.set_trace()
     return res['release']
 
 
@@ -258,7 +257,10 @@ def get_release_list_from_device(device):
     if 'disc' in res:
         return res['disc']['release-list']
     else:
+        try:
             return res['release-list']
+        except KeyError:
+            return []
 
 
 def parse_date(datestr):
@@ -360,6 +362,9 @@ def main():
             release = prompt_select_release(release_list)
         elif len(release_list) == 1:
             release = release_list[0]
+        else:
+            sys.stderr.write('Could not find a matching MusicBrainz release\n')
+            raise SystemExit(1)
 
     print('Ripping', format_release(release))