Use this function instead of CheckError() when we already know an
error has occurred.

Obsoleted by commit d4d4d621

This reduces overhead for two reasons:

1. calls to Remove() are non-blocking

2. RunDeferred() may work on large chunks at a time, reducing the
   number of RunDeferred() calls

There's no point in passing a LightSong reference here; the callee is
interested only in the URI.

Use walk!=nullptr instead.

Don't need to clear the caller-specified one.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

This is the final piece for full ID3/APE support in archives.

Don't open the ZIP file again and again for each song file.

For exception-safety.