converted assert into exception

A few days I ago I was experimenting for a possible answer to [this question](http://matplotlib.1069221.n5.nabble.com/Re-combine-different-axes-to-make-new-figure-in-matplotlib-td43385.html). While trying

    fig1, ax1 = plt.subplots()
    fig2, ax2 = plt.subplots()
    fig2.add_axes(ax1)
I stumbled into an `AssertionError` without any explanation (I had to get the doc of `add_axes`). 
My understanding is that `assert` should be reserved for debugging, not to throw errors if the user give a wrong parameter. So I've converted them to exception. 

I've also noticed that `assert` is used all over the places. If there are no strong opinions in the next weeks I'll to go through the code and convert them to exceptions.