If the MovieWriterRegistry tries to access a MovieWriter that isn't available, it currently just throws a KeyError.

This change catches that and prints a RuntimeError to let the user know which writer is being accessed, but isn't available.

Discovered by running this in a notebook with an ffmpeg MoveWriter not available:

import matplotlib.pyplot as plt
from matplotlib import animation
from IPython.display import HTML

fig, ax = plt.subplots()

def animate(i): 
    return None

anim = animation.FuncAnimation(fig, animate)
HTML(anim.to_html5_video())