design to make the errors/stack traces less confusing and overwhelming

[tarpas]

We have to avoid printing loads of failure reports because it's overwhelming. This is a general problem with any test runner, but we must not make it worse with testmon. At the moment testmon caches and reports all of the failures. Even when nothing is changed all of the reports are reported again (from cache).

My plan so far is:

• Add all the FAIL/ERROR tests at the end of execution queue after "unstable" tests, even though they are "stable".
• make -x default, so that after the first failure, the execution stops

fyi @blueyed
re #135

[tarpas]

A lot of new functionality in v1.4.0. Let's see how ti affects usability.