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/TagFile.cxx

@@ -47,11 +47,11 @@ public:
 		 handler(_handler),
 		 is(nullptr) {}
 
-	bool ScanFile(const DecoderPlugin &plugin) noexcept {
+	bool ScanFile(const DecoderPlugin &plugin) {
 		return plugin.ScanFile(path_fs, handler);
 	}
 
-	bool ScanStream(const DecoderPlugin &plugin) noexcept {
+	bool ScanStream(const DecoderPlugin &plugin) {
 		if (plugin.scan_stream == nullptr)
 			return false;
 
@@ -73,14 +73,14 @@ public:
 		return plugin.ScanStream(*is, handler);
 	}
 
-	bool Scan(const DecoderPlugin &plugin) noexcept {
+	bool Scan(const DecoderPlugin &plugin) {
 		return plugin.SupportsSuffix(suffix) &&
 			(ScanFile(plugin) || ScanStream(plugin));
 	}
 };
 
 bool
-ScanFileTagsNoGeneric(Path path_fs, TagHandler &handler) noexcept
+ScanFileTagsNoGeneric(Path path_fs, TagHandler &handler)
 {
 	assert(!path_fs.IsNull());
 
@@ -100,7 +100,7 @@ ScanFileTagsNoGeneric(Path path_fs, TagHandler &handler) noexcept
 
 bool
 ScanFileTagsWithGeneric(Path path, TagBuilder &builder,
-			AudioFormat *audio_format) noexcept
+			AudioFormat *audio_format)
 {
 	FullTagHandler h(builder, audio_format);