First of all, I did not know the hostage but all my thoughts are with him and his family.
Secondly, David and I flagged the issue on the Mali threat…

About the failure: I would not call it as a failure but as a desperate measure that did not success. Facts are that the hostage was 78 years old and had a hearth disease. Viewing the fact that AQIM did not want to supply him with his drugs and that in the first and last message he gave he mentioned dramatic conditions of retentions, there are good chances that he was already dead several days or weeks before the attack. But this will be confirmed later, I believe.

Concerning Mike’s question, I think that we are here in the case of legitimate use of force due to the imminence of a deadly threat on an innocent victim which goes by both French civilian law and military law.
According to the French Ministry of Defence law, rules and regulation of war, the use of force seems 200% legitimate as an acute imminent threat over either military or civilian French personal or individual has been identified.
The difficulty comes on the fact that the events took place in a foreign country. I am not in the secrets of the bilateral security cooperation agreements between France and Mauritania but I believe this took place accordingly that/those agreements.
In the blog Secret Defence from the daily newspaper Liberation, there is a detail explanation of how the operation took place. French special ops were involved only because there were suspicions of the possible presence of the hostage (but no acute and confirmed presence of him).
Also, it has to be incorporated into a larger picture and linked with the 2 French hostages in Afghanistan case. (That I do not know well). So I believe the message from our president (Which was not really thrilling in terms of dialectic but rather pretty clear for hostage takers) was addressing a much larger audience than just AQIM.
To make a long story short, France had several citizens taken hostages in the past year in Africa and their liberation went fine. With or without COS (the French Special Ops) involvement… I do not know.
Also, 2 years ago (If I do not mistake), 2 French citizens were assassinated by AQIM supporters in Mauritania, which can be considered as an act of war by AQIM. Or at least can be considered as a precedent. Therefore, but I have to make some research on this, technically, France was already at war (even if not considered as such) with AQIM.
Concerning the police/military cooperation in hostage/terrorism management operations:
It is actually true that France tend to treat such situation as a police matter. But the use of military personal and capacity is something which is common. The abduction of the Ponent sailors by Somali pirates and their liberation by military personal is one of the many precedents.
Actually, the standard procedure is to use military capacity (through GIGN from gendarmerie or COS) against terrorist but under a civil legal action. (It is raw, I know). This is quite detailed in Mr Bigo’s book Mike mentioned in the threat on conflict resolution vs "material support for terrorism".

I hope to be able to come with a more detailed response concerning the legal extension of this.

PS: comments on the French president are quite accurate. But it is a personal opinion.