throw timeout error not as `XHRError Exn.Error`, but as separate error

[srghma]

• It's frustrating to have to look at a message of XHRError Exn.Error to see if it's timeout error OR some other error

Describe the solution you'd like

change

data Error
  = RequestContentError String
  | ResponseBodyError ForeignError (Response Foreign)
  | XHRError Exn.Error

to

data Error
  = RequestContentError String
  | ResponseBodyError ForeignError (Response Foreign)
  | XHRError Exn.Error
  | TimeoutError