fix(admin-ui): show message based on error code in Admin UI

[duttarnab]

Title: Something went wrong when attempting to communicate with the backend.

503 -- Gluu Flex Admin UI is not getting any response from the backend (Jans Config Api).

For the rest of the error code displays the error code and message obtained from the API.

Display format

{Title} (in bold)

{Error message}
HTTP Response code : {Error code}