Meaningful error message if predict is called before fit (for #462)

R/R/prophet.R

@@ -1305,6 +1305,9 @@ fit.prophet <- function(m, df, ...) {
 #'
 #' @export
 predict.prophet <- function(object, df = NULL, ...) {
+  if (is.null(object$history)) {
+    stop("Model must be fit before predictions can be made.")
+  }
   if (is.null(df)) {
     df <- object$history
   } else {

python/fbprophet/forecaster.py

@@ -1147,6 +1147,9 @@ class Prophet(object):
         -------
         A pd.DataFrame with the forecast components.
         """
+        if self.history is None:
+            raise Exception('Model must be fit before predictions can be made.')
+
         if df is None:
             df = self.history.copy()
         else: