Temporary HTTP errors should not cached by mw.api.getToken.

Steps to reproduce

• Load MediaWiki
• Switch browser to offline mode
• Enter

var api = new mw.Api();
api.getToken( 'foo' )
.done( function ( data ) {
console.log( 'done', data );
} )
.fail( function ( error ) {
console.log( 'fail', error );
} );

fail http

• Switch browser to online mode
• Enter

api.getToken( 'foo' )
.done( function ( data ) {
console.log( 'done', data );
} )
.fail( function ( error ) {
console.log( 'fail', error );
} );

fail fail

expected result

fail token-missing

Version: 1.24rc
Severity: normal