Replace spurious exit call by raising exceptions.

src/tscut/exceptions.py

@@ -18,5 +18,5 @@ class ExternalToolError(TSCutError):
 class InvalidMediaError(TSCutError):
     """Raised when input media cannot be processed."""
 
-class UnimplementedFeature(TSCutError):
+class UnimplementedFeatureError(TSCutError):
     """Raised when a rare feature is encountered and not yet implemented"""