Don't swallow errors with RNG keys in parameter initialisers.

[copybara-service]

Don't swallow errors with RNG keys in parameter initialisers.

Given the following code snippet:

def f(x):
  x = hk.Conv2D(3, 1)(x)
  return x

f = hk.transform(f)
x = jnp.ones([1, 28, 28])
f.init(None, x)

At HEAD we fail with a confusing error:

ValueError: Parameters cannot be `None`.

After this change we fail with a far more useful:

MissingRNGError: You must pass a non-None PRNGKey to init and/or apply if you
make use of random numbers.