logging fpr disc-id improved

CDDB.py

@@ -140,4 +140,5 @@ def discid():
         try:
             return subprocess.check_output(discid_cmd).decode("utf-8").strip().replace(" ", "+")
         except subprocess.CalledProcessError as e:
+            logger.error("disc-id command results in an exception")
             return None

common.py

@@ -69,4 +69,6 @@ def get_filetype(full_path):
 def get_disc_device():
     rv = discid.get_default_device()
     logger.debug("Disc device is %s", repr(rv))
+    if not os.path.exists(rv):
+        logger.warning("Disk device does not exist!")
     return rv