Improve error message for multiple formatters using STDOUT

Since the formatters used can be added from several places, from ```-Dcucumber.options``` on the command line, from the runner class ```@CucumberOptions```, from the base class ```@CucumberOptions```, it can be possible to have two formatters using STDOUT without realising it (see [mailing list post](https://groups.google.com/d/msg/cukes/8bZd6dWnSwc/fSQgztzCjyUJ)).

It would be helpful if the error message lists the two formatter strings that are conflicting.

Relates to #643.