Adding APIManagerErrorConstants which has all error codes.

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@178183 a5903396-d722-0410-b921-86c7d4935375
Adding Proper error handle implementation.

git-svn-id: https://svn.wso2.org/repos/wso2/carbon/platform/trunk@178184 a5903396-d722-0410-b921-86c7d4935375