Raise error when pending is provided a block.

- Passing a block is explicitly deprecated at this point, so it's
  invalid syntax; invalid method calls should raise `ArgumentError`

  Ruby let's you provide a block to anything, leaving it up to called
  method to decide if it should be used. While this is a feature of the
  language, silently ignored blocks tend to be a source of confusion and
  wasted time. This is especially true when documentation states a block
  can be provided. In this case so many blog posts and other non-RSpec
  controlled docs state `pending` can take a block. Since we can't
  update everyone's docs, it's helpful to be explicit that this is now
  incorrect behavior.

  Printing a warning that this behavior is deprecated and ignoring the
  block is another option. It is my opinion that warnings are simply too
  easily ignored. Thus, an error is preferred to explicitly inform the
  user that this is invalid RSpec language syntax.

- Ignoring the block will fundamentally change the behavior of the
  example in most cases (i.e. the rest of the example will be empty)

  Raising the exception before the example is marked as `pending` is
  desired. This is desired for several reasons:

    * the `pending` syntax is incorrect; QED the example isn't pending
    * it calls attention to the user that the block should be moved
    * this provides a better error message than if the example was
      marked pending and the ArgumentError was raised:

          1) This is my describe block description FIXED
             Expected pending 'My provided pending reasoning' to fail.
             No Error was raised.