phantom export throws error if reveal.js isn't present

plugin/print-pdf/print-pdf.js

@@ -31,9 +31,11 @@ probePage.open( inputFile, function( status ) {
 		return Reveal.getConfig();
 	} );
 
+	if( config ) {
+
 		printPage.paperSize = {
-		width: config.width * ( 1 + config.margin ),
-		height: config.height * ( 1 + config.margin ),
+			width: Math.floor( config.width * ( 1 + config.margin ) ),
+			height: Math.floor( config.height * ( 1 + config.margin ) ),
 			border: 0
 		};
 
@@ -46,6 +48,14 @@ probePage.open( inputFile, function( status ) {
 			}, 1000 );
 		} );
 
+	}
+	else {
+
+        console.log( 'Export PDF: Unable to read reveal.js config. Make sure the input address points to a reveal.js page.' );
+        phantom.exit(1);
+
+    }
+
 } );