[wptrunner] Re-enable return code

In a previous commit [1] the WPT CLI was modified to ignore the return
value of the `wptrunner.start` method. Although this behavior is
convenient in cases where there is no expectation data, it obscures
errors that are relevant in any context.

Specifically, the `start` method returns a non-zero value when no tests
are run, but this is a reliable indicator of an erroneous configuration.
Because the `wpt run` command is used to validate the infrastructure of
the web-platform-tests project, ignoring this case allows
mis-configurations to go unnoticed [2].

Re-enable observance of the value returned by `wptrunner.start` and
update the code generated for the TaskCluster service to ignore this
value.

[1] 5a1b036
[2] #10721