Better error message when mixing N dimension elements with M dimension mesh

[jnbrunet]

In python, doing this:

cells = np.array([[1,2,3], [4,5,6], [7,8,9]])
mesh.add_domain("triangles", Triangle(Caribou._2D, Caribou.Quadratic), cells)

will fail since we are passing 3D coordinates to a 2D domain. However, the output error message from pybind11 is unreadable.