when test fails, indicate which step failed more clearly

[astampoulis]

If I have this protocol:

- foo
- bar
- foo

And this script:

foo
bar
baz

I would like the error reporting to look something like this:

- foo
- bar
vvvvvvvv
- foo
^^^^^^^
error happened here

(I just mean something that points to the exact line, e.g. a number or something like that.)