Don't send SignalExceptions to Sentry.

These don't represent app issues so let's not log them to Sentry.

Some teams have all their Sentry events going to Slack and these have
been making the noise even worse than it need be.

lib/govuk_app_config/govuk_error/configuration.rb

@@ -45,6 +45,7 @@ def set_up_defaults
         "Sinatra::NotFound",
         "Slimmer::IntermittentRetrievalError",
         "Sidekiq::JobRetry::Skip",
+        "SignalException",
       ]
 
       # This will exclude exceptions that are triggered by one of the ignored