Fail messages without logfile set cause traceback

Summary

widget.py:L694 assumes that all FAIL messages coming from jobs (message.scheduler=True) have logfile set.

This is not asserted anywhere... This should be checked in the message API.

Steps to reproduce

Produce a fail message from within a job without setting logfile= in the constructor

What is the current bug behavior?

Traceback when buildstream attempts to get the last 20 lines in the log.

What is the expected correct behavior?

You should get an Assertion error from the message API.

Possible fixes

Other relevant information

• BuildStream version affected: master

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