Recommend 'tensorflow-probability[tf]' extra on tf-keras import failure

* In addition to recommending users install TensorFlow dependencies piecemeal, also
  recommend using the 'tf' extra to install all required dependencies to use
  tensorflow-probability with tensorflow.
* Split this off from the rest of the error message as it is already too long.

tensorflow_probability/python/__init__.py

@@ -83,7 +83,8 @@ def _validate_tf_environment(package):
             'installed by default when you install TensorFlow Probability. '
             'This is so that JAX-only users do not have to install TensorFlow '
             'or TF-Keras. To use TensorFlow Probability with TensorFlow, '
-            'please install the tf-keras or tf-keras-nightly package.\n\n')
+            'please install the tf-keras or tf-keras-nightly package.\n'
+            'This can be be done through the tensorflow-probability[tf] extra.\n\n')
       raise