db/simple/Song: allow LoadFile(), UpdateFile() to throw

Preparing to move logger calls out of lower-level libaries, and
propagating error details to the caller instead.

src/db/update/UpdateSong.cxx

@@ -33,7 +33,7 @@ inline void
 UpdateWalk::UpdateSongFile2(Directory &directory,
 			    const char *name, const char *suffix,
 			    const StorageFileInfo &info) noexcept
-{
+try {
 	Song *song;
 	{
 		const ScopeDatabaseLock protect;
@@ -61,6 +61,7 @@ UpdateWalk::UpdateSongFile2(Directory &directory,
 	if (song == nullptr) {
 		FormatDebug(update_domain, "reading %s/%s",
 			    directory.GetPath(), name);
+
 		auto new_song = Song::LoadFile(storage, name, directory);
 		if (!new_song) {
 			FormatDebug(update_domain,
@@ -89,6 +90,10 @@ UpdateWalk::UpdateSongFile2(Directory &directory,
 
 		modified = true;
 	}
+} catch (...) {
+	FormatError(std::current_exception(),
+		    "error reading file %s/%s",
+		    directory.GetPath(), name);
 }
 
 bool