Add error events to input framework.

This change introduces error events for Table and Event readers. Users
can now specify an event that is called when an info, warning, or error
is emitted by their input reader. This can, e.g., be used to raise
notices in case errors occur when reading an important input stream.

Example:

event error_event(desc: Input::TableDescription, msg: string, level: Reporter::Level)
	{
	...
	}

event bro_init()
	{
	Input::add_table([$source="a", $error_ev=error_event, ...]);
	}

For the moment, this converts all errors in the Asciiformatter into
warnings (to show that they are non-fatal) - the Reader itself also has
to throw an Error to show that a fatal error occurred and processing
will be abort.

It might be nicer to change this and require readers to mark fatal
errors as such when throwing them.

Addresses BIT-1181

src/logging/WriterBackend.h

@@ -38,7 +38,7 @@ public:
 	 * @param name A descriptive name for writer's type (e.g., \c Ascii).
 	 *
 	 */
-	WriterBackend(WriterFrontend* frontend);
+	explicit WriterBackend(WriterFrontend* frontend);
 
 	/**
 	 * Destructor.
@@ -248,8 +248,11 @@ public:
 	bool FinishedRotation();
 
 	// Overridden from MsgThread.
-	virtual bool OnHeartbeat(double network_time, double current_time);
-	virtual bool OnFinish(double network_time);
+	bool OnHeartbeat(double network_time, double current_time) override;
+	bool OnFinish(double network_time) override;
+
+	// Let the compiler know that we are aware that there is a virtual info function in the base
+	using MsgThread::Info;
 
 protected:
 	friend class FinishMessage;