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

[matthewfeickert]

• 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.

As this amends https://github.com/tensorflow/probability/commit/988f02323f16419086d761cc6cdd50b94aa03257 tagging @jburnim for review.

[matthewfeickert]

Note the pylint errors are not associated with my PR:

$ pylint -j2 --rcfile=testing/pylintrc tensorflow_probability/python/__init__.py
pylint: Command line or configuration file:1: UserWarning: 'StandardError' is not a proper value for the 'overgeneral-exceptions' option. Use fully qualified name (maybe 'builtins.StandardError' ?) instead. This will cease to be checked at runtime when the configuration upgrader is released.
pylint: Command line or configuration file:1: UserWarning: 'Exception' is not a proper value for the 'overgeneral-exceptions' option. Use fully qualified name (maybe 'builtins.Exception' ?) instead. This will cease to be checked at runtime when the configuration upgrader is released.
pylint: Command line or configuration file:1: UserWarning: 'BaseException' is not a proper value for the 'overgeneral-exceptions' option. Use fully qualified name (maybe 'builtins.BaseException' ?) instead. This will cease to be checked at runtime when the configuration upgrader is released.
************* Module tensorflow_probability.python
W4901: 49 _validate_tf_environment: Deprecated module 'distutils.version' [deprecated-module]

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Your code has been rated at 9.80/10

[ColCarroll]

thanks for being on top of this, @matthewfeickert !

[matthewfeickert]

Thank you for the fast review, @ColCarroll! Also thanks to you and the rest of the team for tfp in general!

[ColCarroll]

it was all @jburnim on getting the code in and merged efficiently. i'm just the bearer of gratitude 🤗