decoder/Plugin: allow scan_{file,stream}() to throw

Bug #915 is about an I/O exception thrown where none was allowed,
leading to crash via std::terminate().  However, instead of catching
and logging the error inside the decoder plugin, it should be able to
propagate the I/O error to the MPD core, so MPD can avoid trying other
decoder plugins.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/915

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -424,7 +424,7 @@ VisitOpusDuration(InputStream &is, OggSyncState &sync, OggStreamState &stream,
 }
 
 static bool
-mpd_opus_scan_stream(InputStream &is, TagHandler &handler) noexcept
+mpd_opus_scan_stream(InputStream &is, TagHandler &handler)
 {
 	InputStreamReader reader(is);
 	OggSyncState oy(reader);