Fix crash

[vext01]

This fixes the crash we saw yesterday:

Message from krun running on bencher5:

Fatal Krun error: 'NoneType' object has no attribute 'eta_estimates'
  File "krun/krun.py", line 231, in main
    inner_main(mailer, on_first_invocation, config, args)
  File "krun/krun.py", line 357, in inner_main
    sched.run()
  File "/home/kruninit/warmup_experiment/krun/krun/scheduler.py", line 531, in run
    extra_env=self._make_post_cmd_env(results)
  File "/home/kruninit/warmup_experiment/krun/krun/scheduler.py", line 473, in _make_post_cmd_env
    eta_val = self.get_overall_time_estimate_formatter(results).finish_str
  File "/home/kruninit/warmup_experiment/krun/krun/scheduler.py", line 462, in get_overall_time_estimate_formatter
    self.get_estimated_overall_duration(results))
  File "/home/kruninit/warmup_experiment/krun/krun/scheduler.py", line 447, in get_estimated_overall_duration
    per_exec = self.get_estimated_exec_duration(key, results)
  File "/home/kruninit/warmup_experiment/krun/krun/scheduler.py", line 438, in get_estimated_exec_duration
    previous_exec_times = results.eta_estimates.get(key)

I'm not sure why my testing didn't find this when I was implementing the re-run logic, but anyway, I've re-tested carefully and I think this is good.

The first commit is the fix, the message explains what went wrong before.

The second commit is just some defensive programming.

OK?

[ltratt]

I'm fine with this except assert False is an icky idiom IMHO (asserts are removed by -O for example): we should do something like raise RuntimeError.

[vext01]

I didn't know about -O (but it's quite amusing somehow in the case of CPython).

How about a call to fatal instead?

[ltratt]

I'd keep it simple: since it's not that likely to happen, raise RuntimeError or equivalent seems easy enough.

[vext01]

Here we go. See also #324 as there are quite a few assertions in-tree (I don't want to fix those immediately though).

[ltratt]

OK, squash and I'll merge.

[vext01]

splat.

[vext01]

Fixed typo.