Return true/false for validation function

freqtrade/optimize/__init__.py

@@ -78,7 +78,7 @@ def get_timeframe(data: Dict[str, DataFrame]) -> Tuple[arrow.Arrow, arrow.Arrow]
 
 
 def validate_backtest_data(data: Dict[str, DataFrame], min_date: datetime,
-                           max_date: datetime, ticker_interval_mins: int) -> None:
+                           max_date: datetime, ticker_interval_mins: int) -> bool:
     """
     Validates preprocessed backtesting data for missing values and shows warnings about it that.
 
@@ -89,10 +89,13 @@ def validate_backtest_data(data: Dict[str, DataFrame], min_date: datetime,
     """
     # total difference in minutes / interval-minutes
     expected_frames = int((max_date - min_date).total_seconds() // 60 // ticker_interval_mins)
+    found_missing = False
     for pair, df in data.items():
         if len(df) < expected_frames:
+            found_missing = True
             logger.warning('%s has missing frames: expected %s, got %s',
                            pair, expected_frames, len(df))
+    return found_missing
 
 
 def load_tickerdata_file(