improved function execution failure response

- let the LLM know in a more clear way that a function execution failed.
- This is for situations where the function didn't error during execution, but it couldn't be called at all.

lib/function.ex

@@ -222,7 +222,7 @@ defmodule LangChain.Function do
     rescue
       err ->
         Logger.error("Function #{function.name} failed in execution. Exception: #{inspect(err)}")
-        {:error, inspect(err)}
+        {:error, "ERROR: #{inspect(err)}"}
     end
   end