Add option to not fail the build when a processing exception occurs

[tomdesair]

Currently there is already the failOnServiceError option which will prevent the build from failing if there is a network connection problem.

However, we never want the Coveralls plugin to fail the build when submitting data (e.g. when the API token is missing or invalid). Therefore we need a failOnProcessingError option, which by default is true. If the option is set to false, Processing exceptions should only result in a warning.

[tomdesair]

I've created a pull-request for this here #123