Improve exception management.

Remove almost all std::exit calls.

Now, one should use one of the provided exceptions.

- NormalError: an error which is related to the use of the code. It is
not related to a bug

- NotImplementedError: an error related to a functionnality that is
not avaliable yet

- UnexpectedError: an error related to some unexpected state. This
is related to a bug

Closes #13