fs/io/BufferedReader: new class to replace class TextFile

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

src/StateFile.cxx

@@ -103,10 +103,10 @@ StateFile::Read()
 
 	FormatDebug(state_file_domain, "Loading state file %s", path_utf8.c_str());
 
-	TextFile file(path);
+	Error error;
+	TextFile file(path, error);
 	if (file.HasFailed()) {
-		FormatErrno(state_file_domain, "failed to open %s",
-			    path_utf8.c_str());
+		LogError(error);
 		return;
 	}