Improve the error handling of the template composer

When accessing the template composer the first time it is blocking the cms UI with a loading dialog when the HST is not responding and the SSO handshake fails. We need a timeout and give some feedback to the user about the status so the user can decide to request again or switch to a different perspective.

Further more would lead a failing HST meta-data requests to an undefined state, so the user should be aware of it.