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

scripts/base/frameworks/reporter/main.bro

@@ -17,22 +17,15 @@ export {
 	## The reporter logging stream identifier.
 	redef enum Log::ID += { LOG };
 
-	## An indicator of reporter message severity.
-	type Level: enum {
-		## Informational, not needing specific attention.
-		INFO,
-		## Warning of a potential problem.
-		WARNING,
-		## A non-fatal error that should be addressed, but doesn't
-		## terminate program execution.
-		ERROR
-	};
-
 	## The record type which contains the column fields of the reporter log.
 	type Info: record {
 		## The network time at which the reporter event was generated.
 		ts:       time   &log;
 		## The severity of the reporter message.
+		## Levels are INFO for Informational, not needing specific attention,
+		## WARNING for Warning of a potential problem,
+		## and ERROR for A non-fatal error that should be addressed, but doesn't
+		## terminate program execution.
 		level:    Level  &log;
 		## An info/warning/error message that could have either been
 		## generated from the internal Bro core or at the scripting-layer.