Unspecific 500 Server Error when adding already existing pending contact

@grote found python-briar-wrapper#19 (closed) where he encountered problems when re-adding an already existing pending contact.

I tried to somehow handle those error in briar_wrapper, but the response by Briar Headless is always just 500: Internal server error. I get this when adding a pending contact after

• it has already been added seconds before and the peer is offline
• it has already been added more than 4 days before (manually changed system time) and the peer was offline all the time
• the pending contact got already added days before and left the pending state
• the link is invalid (I think so, just changed some characters)
• and for sure there are more errors

The problem is that in this line contactManager.addPendingContact gets called without catching its exceptions. Looking at its implementation, there are at least DbException, FormatException, GeneralSecurityException that get thrown.