widget.py: Require message.logfile for log widget

Prevent traceback in the case which a FAIL message is produced without a
logfile being specified.

Fixes: #619

buildstream/_frontend/widget.py

@@ -679,7 +679,7 @@ class LogLine(Widget):
             text += '\n'
             extra_nl = True
 
-        if message.scheduler and message.message_type == MessageType.FAIL:
+        if message.logfile and message.scheduler and message.message_type == MessageType.FAIL:
             text += '\n'
 
             if self.context is not None and not self.context.log_verbose: