fix: remind user to install ffmpeg when an audio file is not found

The error is usually misleading: a file not found triggers a click error
long before the exception in audio_utils.py more likely means ffprobe
and ffmpeg are not found.

readalongs/audio_utils.py

@@ -123,6 +123,6 @@ def read_audio_from_file(path: str) -> AudioSegment:
     except Exception as e:
         # need repr(e) here instead of e since these exceptions don't all have messages
         raise RuntimeError(
-            "Error reading audio file %s: %s. Please make sure you "
-            "provide a valid audio file." % (path, repr(e))
+            "Error reading audio file %s: %s. Please provide a valid audio file and "
+            "make sure ffmpeg is installed." % (path, repr(e))
         ) from e