Error reporting

• Most FCM routines return NULL on failure
• const char *fcm_error_message() - returns a textual description of the most recent error

• FCM_FILE fcm = fcm_open(filename);
  if (fcm == NULL)
      fprintf(stderr, "FCM error: %s\n", fcm_error_message());

• fcm_error_code() returns a numeric code:
  • FCM_ERR_FORMAT - file format error
  • FCM_ERR_NOT_FOUND - object not found
  • ... others, see fcm.h for full list.