errors: calculate errors in neural network for derivatives. Errors...
In wiper8/AI: Provide AI subject functions.

calculate errors in neural network for derivatives. Errors are derivatives of the cost function with regards to the z (XW+b).

1	errors(nn, acti, inputs, target, Loss_fun = TRUE, policy_linear_output = TRUE)

`nn`	neural network object.
`acti`	list of matrices of activations (from activations function).
`inputs`	matrix of observations with every input.
`target`	matrix of observations with every real target.
`Loss_fun`	logical : if the function to maximize/minimize is a Loss function. if FALSE, it is considered to be a linear target.
`policy_linear_output`	logical : if the policy output in DDPG is linear.