Skip voices that aren't installed yet in MaryTTS /voices endpoint

mimic3_tts/download.py

@@ -141,6 +141,11 @@ def download_voice(
             ) from e
 
 
+def is_voice_downloaded(voice_location: str) -> bool:
+    """True if voice has been downloaded locally"""
+    return voice_location and Path(voice_location).exists()
+
+
 # -----------------------------------------------------------------------------