newrelic: refactor error logging.

* Factor out New Relic logging to DanbooruLogger class.
* Log all exceptions to New Relic, not just statement timeouts.

app/logical/danbooru_logger.rb

@@ -0,0 +1,12 @@
+class DanbooruLogger
+  def self.log(exception, expected: false, **params)
+    if !expected
+      backtrace = Rails.backtrace_cleaner.clean(exception.backtrace).join("\n")
+      Rails.logger.error("#{exception.class}: #{exception.message}\n#{backtrace}")
+    end
+
+    if defined?(::NewRelic)
+      ::NewRelic::Agent.notice_error(exception, expected: expected, custom_params: params)
+    end
+  end
+end