Clearer errors about invalid file types.

## Why?

 * The error on a bad freesound/sample does not tell you about the valid file types.
 * We have to wait till after the download of the freesound file to find out its invalid.

## Solution

1. List the valid file types in the error message.
2. Check the file type before download from freesound.

I don't know how complete the type info is on Freesound so if type is not present pass it through (and the buffer loading will raise an error).

## Goal

Help newcomers quickly learn what they can and cannot use from freesound.