Whatever happen when saving backlinks it should never break the document save

Each time a document is saved, its backlinks are calculated just before saving it. If anything goes wrong when extracting them the document save crash.

Backlinks are in no way critical enough to kill the document save and there should be a protection around it. Ideally all this code should be moved in a document save listener IMO, it not needed to have it tied that much to the document saving.