don't return 0 on exec failure

additionally return EX_UNAVAILABLE [0] if the child terminated
abnormally or returned non-zero.

[0]: https://man.freebsd.org/cgi/man.cgi?sektion=3&query=sysexits

Closes: #271

src/scrot.c

@@ -633,11 +633,10 @@ static void scrotExecApp(Imlib_Image image, struct tm *tm, char *filenameIM,
 {
     char *execStr = imPrintf(opt.exec, tm, filenameIM, filenameThumb, image);
     int ret = system(execStr);
-
     if (ret == -1)
-        warn("The child process could not be created");
-    else if (WIFEXITED(ret) && WEXITSTATUS(ret) == 127)
-        warnx("scrot could not execute the command: %s", execStr);
+        err(EXIT_FAILURE, "The child process could not be created");
+    else if (!WIFEXITED(ret) || WEXITSTATUS(ret) != 0)
+        exit(69 /* EX_UNAVAILABLE */ );
     free(execStr);
 }