Exit webrunner with 1 instead of 0 on assert failure

[ivansabik]

Not sure if this related to PyWebRunner, but was doing some tests with Travis and jumped into an issue with running YAML scripts with webrunner. Namely if an assert fails, Travis still exists with success status:

Here's the Travis run in question:
https://travis-ci.org/ivansabik/PyWebRunnerExamples#L186-L211

[ivansabik]

By doing a quick skim of the code I can think of two possibilities for now, adding sys.exit(1) here:

• https://github.com/IntuitiveWebSolutions/PyWebRunner/blob/master/PyWebRunner/WebRunner.py#L456
• https://github.com/IntuitiveWebSolutions/PyWebRunner/blob/master/PyWebRunner/WebRunner.py#L474
• https://github.com/IntuitiveWebSolutions/PyWebRunner/blob/master/PyWebRunner/WebRunner.py#L484

Or adding a new arg to the stop method something like def stop(self, sys_exit_error=True): https://github.com/IntuitiveWebSolutions/PyWebRunner/blob/master/PyWebRunner/WebRunner.py#L218