Better error handling

[bonsairobo]

For example do_test_on_package does not return a Result, but instead it just manages a failed bool and keeps running through the code without early returns. We end up with sub-standard error reporting where the process fails and just prints:

ERROR cargo_fslabscli: Could not execute command: tests failed

Even with -vvvv that's all we get.

We should do some refactoring to make sure errors are not dropped so quickly.

[bonsairobo]

OK I just realized we can actually get more info about failures from the junit report. Still though; it's not obvious where to look.

[loispostula]

@bonsairobo what fixes are you proposing?

[bonsairobo]

I think the error that causes the tool to exit should actually contain some context. It shouldn't be necessary to look at the junit report to learn what caused the error.