MDL-14591 - adding missing portfolio error string

lang/en_utf8/portfolio.php

@@ -44,6 +44,7 @@
 $string['nopermissions'] = 'Sorry but you do not have the required permissions to export files from this area';
 $string['nonprimative'] = 'A non primative value was passed as a callback argument to portfolio_add_button.  Refusing to continue.  The key was $a->key and the value was $a->value';
 $string['notexportable'] = 'Sorry, but the type of content you are trying to export is not exportable';
+$string['notimplemented'] = 'Sorry, but you are trying to export content in some format that is not yet implemented ($a)';
 $string['nouploaddirectory'] = 'Could not create a temporary directory to package your data into';
 $string['portfolio'] = 'Portfolio';
 $string['portfolios'] = 'Portfolios';