nsq_to_file: check and exit on errors for file-related operations

apps/nsq_to_file/file_logger.go

@@ -159,10 +159,19 @@ func (f *FileLogger) Close() {
 	}
 
 	if f.gzipWriter != nil {
-		f.gzipWriter.Close()
+		err := f.gzipWriter.Close()
+		if err != nil {
+			log.Fatalf("ERROR: failed to close GZIP writer: %s", err)
+		}
+	}
+	err := f.out.Sync()
+	if err != nil {
+		log.Fatalf("ERROR: failed to fsync output file: %s", err)
+	}
+	err = f.out.Close()
+	if err != nil {
+		log.Fatalf("ERROR: failed to close output file: %s", err)
 	}
-	f.out.Sync()
-	f.out.Close()
 
 	// Move file from work dir to output dir if necessary, taking care not
 	// to overwrite existing files