Cleanup of error handling

Introduced consistent way of handling errors and printing error messages.

Also upgraded some warnings to errors.

src/main.c

@@ -32,21 +32,18 @@ int main(int argc, char *argv[])
 {
     int status;
 
+    /* Install error exit handler */
+    atexit(&error_exit);
+
     /* Parse options */
     parse_options(argc, argv);
 
     /* Configure output terminal */
     configure_stdout();
 
-    /* Install error exit handler */
-    atexit(&error_exit);
-
     /* Install log exit handler */
     atexit(&log_exit);
 
-    /* Restore output terminal on exit */
-    atexit(&restore_stdout);
-
     /* Create log file */
     if (option.log)
         log_open(option.log_filename);