Add Fail Thresholds

[Ardesco]

After we have moved the reporting into the verify phase we could also add some simple fail thresholds.

If you only care about a % of requests working we can have a look at the number of failed requests and work out a success percentage and trigger a failure if it's lower than the fail threshold.

[somayaj]

Any chance we can add the response times (avg, min, max) as thresholds as well?