Prevent exception in offset find

Whipper offset find needs a (remote) matching AccurateRip entry to perform its task and confirm the drive offset: if none is found, return early to prevent exception.

Fixes #208.

whipper/command/offset.py

@@ -95,6 +95,7 @@ CD in the AccurateRip database."""
         except accurip.EntryNotFound:
             logger.warning("AccurateRip entry not found: drive offset "
                            "can't be determined, try again with another disc")
+            return
 
         if responses:
             logger.debug('%d AccurateRip responses found.' % len(responses))