Enhance available voices check

ukrainian_tts/tts.py

@@ -60,9 +60,10 @@ class TTS:
         else:
             stress = False
         if voice not in [option.value for option in Voices]:
-            raise ValueError(
-                f"Invalid value for voice selected! Please use one of the following values: {', '.join([option.value for option in Voices])}."
-            )
+            if voice not in self.xvectors.keys():
+                raise ValueError(
+                    f"Invalid value for voice selected! Please use one of the following values: {', '.join([option.value for option in Voices])}."
+                )
 
         text = preprocess_text(text)
         text = sentence_to_stress(text, stress_with_model if stress else stress_dict)