Avoiding infinite loops when an error message handlers triggers errors

itself.

If an error is triggered inside one of the reporter_* handlers, the
message about that will now fall back to stderr.

src/EventHandler.h

@@ -39,6 +39,11 @@ public:
 	void SetUsed()          { used = true; }
 	bool Used()             { return used; }
 
+	// Handlers marked as error handlers will not be called recursively to
+	// avoid infinite loops if they trigger a similar error themselves.
+	void SetErrorHandler()          { error_handler = true; }
+	bool ErrorHandler()             { return error_handler; }
+
 	const char* Group()	{ return group; }
 	void SetGroup(const char* arg_group)
 				{ group = copy_string(arg_group); }
@@ -57,6 +62,7 @@ private:
 	FuncType* type;
 	bool used;		// this handler is indeed used somewhere
 	bool enabled;
+	bool error_handler;	// this handler reports error messages.
 
 	declare(List, SourceID);
 	typedef List(SourceID) receiver_list;