message: Add further error validation to linear unit activations.
diff: @@ -72,8 +72,9 @@ class LeakyReLU(Layer):
  def __init__(self, alpha=0.3, **kwargs):
  super(LeakyReLU, self).__init__(**kwargs)
  if alpha is None:
  - raise ValueError('alpha of leaky Relu layer '
  - 'cannot be None. Required a float')
  + raise ValueError('The alpha value of a Leaky ReLU layer '
  + 'cannot be None, needs a float. '
  + 'Got %s' % alpha)
  self.supports_masking = True
  self.alpha = K.cast_to_floatx(alpha)
  @@ -210,7 +211,8 @@ class ELU(Layer):
  def __init__(self, alpha=1.0, **kwargs):
  super(ELU, self).__init__(**kwargs)
  if alpha is None:
  - raise ValueError('alpha of ELU layer ' 'cannot be None. Required a float')
  + raise ValueError('Alpha of an ELU layer cannot be None, '
  + 'requires a float. Got %s' % alpha)
  self.supports_masking = True
  self.alpha = K.cast_to_floatx(alpha)
  @@ -252,6 +254,9 @@ class ThresholdedReLU(Layer):
  def __init__(self, theta=1.0, **kwargs):
  super(ThresholdedReLU, self).__init__(**kwargs)
  + if theta is None:
  + raise ValueError('Theta of a Thresholded ReLU layer cannot be '
  + 'None, requires a float. Got %s' % theta)
  self.supports_masking = True
  self.theta = K.cast_to_floatx(theta)
  