Throw an error if trying to convert a > 1-bit BMP file to XBin

exec/xbimage.js

-// $Id: xbimage.js,v 1.6 2020/04/19 19:52:31 rswindell Exp $
-
 // Utility module for creating and displaying XBin image files.
 
 load('sbbsdefs.js');
@@ -34,6 +32,11 @@ function convert_from_bmp(filename, charheight, fg_color, bg_color, palette, inv
 		return false;
 	}
 
+	if(bmp.infoheader.biBitCount != 1) {
+		alert(format("Bit-count (%d) must be 1 (monochrome)", bmp.infoheader.biBitCount));
+		return false;
+	}
+
 	// Add SAUCE 
 	var sauce = {
 		 title: title ? title : filename