make discid a non required dependency

CDDB.py

 if 'EMAIL' in os.environ:
     (default_user, hostname) = os.environ['EMAIL'].split('@')
 else:
-    default_user = os.environ['USER'] or os.geteuid() or 'user'
+    default_user = os.environ.get('USER') or os.geteuid() or 'user'
     hostname = socket.gethostname() or 'host'
 
 proto = 6

common.py

 import os
-import discid
+import logging
+
+try:
+    from config import APP_NAME as ROOT_LOGGER_NAME
+except ImportError:
+    ROOT_LOGGER_NAME = 'root'
+logger = logging.getLogger(ROOT_LOGGER_NAME).getChild(__name__)
+
+# make module usable without discid dependency
+try:
+    import discid
+except ModuleNotFoundError:
+    logger.warning("Python module discid not installed")
+except OSError:
+    logger.exception("You might install python3-libdiscid")
 
 KEY_ALBUM = 'album'
 KEY_APERTURE = 'aperture'

metadata.py

 except ImportError:
     ROOT_LOGGER_NAME = 'root'
 logger = logging.getLogger(ROOT_LOGGER_NAME).getChild(__name__)
+
+# make module usable without discid dependency
 try:
     import discid
+except ModuleNotFoundError:
+    logger.warning("Python module discid not installed")
 except OSError:
     logger.exception("You might install python3-libdiscid")