create --write-json dir if it doesn't exist

clarify mkdir error message

Author: wiedehopf

readsb.c

@@ -2964,6 +2964,8 @@ int main(int argc, char **argv) {
         Modes.stats_10[j].start = Modes.stats_10[j].end = Modes.stats_current.start;
 
     if (Modes.json_dir) {
+        mkdir_error(Modes.json_dir, 0755, stderr);
+
         char pathbuf[PATH_MAX];
         snprintf(pathbuf, PATH_MAX, "%s/getState", Modes.json_dir);
         mkdir_error(pathbuf, 0755, stderr);

util.c

@@ -584,7 +584,7 @@ char *sprint_uuid(uint64_t id1, uint64_t id2, char *p) {
 int mkdir_error(const char *path, mode_t mode, FILE *err_stream) {
     int err = mkdir(path, mode);
     if (err != 0 && errno != EEXIST && err_stream) {
-        fprintf(err_stream, "mkdir: %s (%s)\n", strerror(errno), path);
+        fprintf(err_stream, "mkdir %s: %s\n", path, strerror(errno));
     }
     return err;
 }