MockContext does not honor "warn=False" which is the default behaviour

[red8888]

I just ran into an issue where my test wasn't catching a bug I had because of this.

I gracefully catch errors:

result = ctx.run("my command", hide=False, echo=True)
if result.exited:
    ... do stuff

Now I forgot to add warn=True here, but wrote my tests with MockContext:

ctx = MockContext(
    run={
        "my command": Result("Mock output", exited=1),
    }
)
... and checked the "do stuff" after the command returned a non-zero exit code worked as expected

My tests were passing without issue because MockContext isn't honoring the default warn=False. I found in production the error was not being caught because I forgot to set warn=True.

What should have happened was:

1. MockContext sees warn=False and pyinvoke immediately exits after the command returns a non-zero exit code
2. My tests fail and I catch the misconfiguration