Exit previously entered sub-context-managers whenever any __enter__() call

throws an exception in tf.name_scope() and tf.variable_scope().

The previous conversion from generator style context managers translated a
nested with block like

    with context_a, context_b:
      yield

into the following pair of __enter__()/__exit__() methods:

    def __enter__(self):
      self.context_a.__enter__()
      self.context_b.__enter__()

    def __exit__(self, *args):
      self.context_b.__exit__(*args)
      self.context_a.__exit__(*args)
      return False

This translation is only correct when self.context_b.__enter__() does not throw
an exception. In the context of tf.name_scope() and tf.variable_scope(),
context_a is the default graph stack, and context_b is the actual scope to
enter. Entering an actual scope throws a ValueError when the scope name is
invalid. In that case, the above implementation leaves __enter__() without
popping the default graph stack. Sub-sequent calls to pop the stack will thus
fail, obscuring the actual exception that was raised in entering the scope.

PiperOrigin-RevId: 182119816