Add setting to choose if the results are errors or warnings

[xt0rted]

Problem matchers can set a default severity level. A new setting should be added that injects a user selectable value in to this.

The allowed values are error and warning, and it defaults to error.

[xt0rted]

This might not be needed for v5.1.225507 and on since the report output seems to include a violation level now. Still worth investigating though.