Download metadata exception

[rullzer]

Exceptions in the download meta data should be handled a bit more graceful.

1. Use built in saml
2. Click download metadata
3. watch it do boom

We should catch this a bit more graceful .

CC: @blizzz @juliushaertl

[PrivatePuffin]

It already has a nice orange "invalid metadata" tag to the right of the button.
Simple fix would be just disabling the button and it would be pretty clear for every one ;)