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/MadDecoderPlugin.cxx

@@ -1051,7 +1051,7 @@ MadDecoder::RunScan(TagHandler &handler) noexcept
 }
 
 static bool
-mad_decoder_scan_stream(InputStream &is, TagHandler &handler) noexcept
+mad_decoder_scan_stream(InputStream &is, TagHandler &handler)
 {
 	MadDecoder data(nullptr, is);
 	return data.RunScan(handler);