Ensure transcoding failures are never cached

By re-raising any exceptions that `transcode` encounters while running
the transcode, it ensures that `set_generated` will also see the
exception, preventing it from caching an incomplete transcode.

supysonic/api/media.py

@@ -123,7 +123,8 @@ def stream_media():
                     if dec_proc != None:
                         dec_proc.kill()
                     proc.kill()
-
+                    raise
+                finally:
                     if dec_proc != None:
                         dec_proc.wait()
                     proc.wait()