This switch aborts the rebar3 run after the first test failure is
reported.

Since Common Test does not support this feature and it can be useful for
interactive development (a demand from one of the teams I work with), I
decided to add the feature as an experimental CT hook.

The hook notices any test failure, and forcefully aborts the current
script execution, but only during the initialization phase of the next
test; this ensure that all other hooks for a post-test failure have the
time to do things right.

The CT logs _will_ be interupted, and hooks of all kinds may also
suffer.

Since this might be a bit tricky to support internally, I will be fine
with a review that results in "no, Fred. Please test this in a project
plugin first" or if I get told to use profiles to mature the feature
(i.e. `rebar3 as failfast ct ...`) with a custom hook declaration.