ability to add custom login pages and error pages

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@173437 a5903396-d722-0410-b921-86c7d4935375
now the custom loginpage and the error page is 

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@173438 a5903396-d722-0410-b921-86c7d4935375