Plugins is terminated with `SIGKILL`

[ehmicky]

A build using this plugin showed:

Plugin exited with exit code null and signal SIGKILL.

This indicates the plugin killed itself with SIGKILL. It seems that this might come from pa11y. Unfortunately, the build logs does not show anything more than this. It is possible that this is a timeout error as described in #5. However in this case, no TimeoutError was thrown.