TGTCheckAction throws Exception with invalid TGT

The existing TGTAction check attempts to find the TGT present in the context from the registry via:

```java
final Ticket ticket = this.centralAuthenticationService.getTicket(tgtId, Ticket.class);
```

This throws exceptions if ticket id not found. This pull handles the catching of that exception.
Actually, this is good idea. Maybe we could make checkstyle enforce that rule. I'll look into that as a separate issue but will update this one as well.