Have to correct myself there: the "special powers" of the French President in a high emergency only allows the suspension of the second chamber of the Parliament (equivalent to the US House of Representatives), not the Senate, and he has to make consultations with the Prime Minsiter before using those measures.

There was some revision of the constitution encoded this summer, and one of the points was that after a month of Presidential "dictatorship" (the word is not formally used but that's the gist of it) a Constitutional Council can be called by the Chairman of the Senate or by petition of sixty parliamentarians, to examine if the reasons for the takeover are still relevant and are the objective of what the President is doing. What happens if they find he has overstepped fair reasons? On that, the revision is silent. I don't think the French President can be legally impeached (don't hold me as a witness on it though); he can't be taken before a normal court during his time in office.