Code simplification.

Be exception-safe.

Prepare full C++ exception support in the decoder thread.

Fixes memory leak after stream failure.  See
https://bugs.musicpd.org/view.php?id=4562

This reverts commit 995cd954.  It was
a bad idea because it caused assertion failures when a decoder plugin
aborted with an exception.

Allow decoders to throw std::runtime_error.

Fixes an assertion failure in tag_ape_scan() which is because
decoder_input_stream_open() returns an "unready" InputStream.

Adds support for stream codecs which havn't been explicitly listed in
ffmpeg_mime_types.

Keep only the last error.  This fixes bogus aborts due to the error
check in decoder_get_virtual_command().

decoder_load_replay_gain() will create the InputStream anyway, so
we're not saving any overhead by opening the InputStream on demand
only.

Give MPD clients access to the error condition.

Fixes memory leak after InputStream::Check() failure.

Remove that surprising rule.