No feedback when pytest run fails

[jitseniesen]

If a pytest run fails early, then the user get almost no feedback. This happened to me when trying to get a coverage report with PR #175 without having pytest-cov installed, but I have seen it also in other situations.

The only thing that happens is that the "Run tests" icon changes back from the red square (stop icon) to the green triangle (run icon). But the top of the unittest pane keeps showing the misleading text "Running tests ...".

I am pretty sure this is a regression.