Ref: https://nvd.nist.gov/vuln/detail/CVE-2018-1000620

Add maxErrorRate option to config.ensure to make Artillery exit with
a non-zero status code if the ratio of failed to successful scenarios
is higher than the value specified.

For example, exit with non-zero code if any errors occur:

config:
  ensure:
    maxErrorRate: 0

Both OS-level errors such as ECONNRESET, and errors returned from
hooks will be taken into account.

Fixes #506

dep: Switch from rolex to driftless

Including these is unneccesary and makes HTML reports much larger
than they should be.

Fixes #510

Fixes #386.

Since the logic in actually handled by the "utils" engine, I've just added the "whileTrue" properties in step definition.

Signed-off-by: Stéphane HULARD <s.hulard@chstudio.fr>

Fixes #500.