The fact that XWiki#exists return false in case of exception is wrong

An exception does not necessarily mean that the document does not exist, and if it's a temporary problem you might end up storing directly or indirectly in some cache that a document does not exist when in fact it does.

To make things even worst, exceptions are not even logged, so you get weird behavior without any kind of hint.

We need to decide if we just add a throws to the existing #exists methods (which is not really a binary breakage AFAIK but could of course cause problem with code really not expecting this method to ever fail) or if we introduce new ones.