Return error message instead of raising on unsupported indicators

When the LLM agent requests an indicator that isn't supported
(e.g. 'agt'), the function was raising a ValueError which crashed
the Market Analyst. Now it returns an error string that tells the
agent which indicators are available, allowing it to self-correct
and retry with a valid indicator.

Applied to both yfinance and alpha_vantage indicator functions.

Fixes #429

tradingagents/dataflows/alpha_vantage_indicator.py

@@ -58,8 +58,11 @@ def get_indicator(
     }
 
     if indicator not in supported_indicators:
-        raise ValueError(
-            f"Indicator {indicator} is not supported. Please choose from: {list(supported_indicators.keys())}"
+        supported = ", ".join(supported_indicators.keys())
+        return (
+            f"Error: Indicator '{indicator}' is not supported. "
+            f"Supported indicators: {supported}. "
+            f"Please retry with one of the supported indicators."
         )
 
     curr_date_dt = datetime.strptime(curr_date, "%Y-%m-%d")

tradingagents/dataflows/y_finance.py

@@ -129,8 +129,11 @@ def get_stock_stats_indicators_window(
     }
 
     if indicator not in best_ind_params:
-        raise ValueError(
-            f"Indicator {indicator} is not supported. Please choose from: {list(best_ind_params.keys())}"
+        supported = ", ".join(best_ind_params.keys())
+        return (
+            f"Error: Indicator '{indicator}' is not supported. "
+            f"Supported indicators: {supported}. "
+            f"Please retry with one of the supported indicators."
         )
 
     end_date = curr_date