This should never happen, but if it does, lets make sure we know

In the past, this has cause things to break in mysterious ways

Change-Id: I2fcf1e2a9cf776a9ab9b3969d9ec552e23120e79

modules/ve/ve.Document.js

@@ -416,6 +416,9 @@ ve.Document.prototype.selectNodes = function ( range, mode ) {
 			}
 		}
 	} while ( end >= left - 1 );
+	if ( retval.length === 0 ) {
+		throw new Error( 'selectNodes epic fail' );
+	}
 	return retval;
 };