conf-parser: never consider it an error if we cannot load a drop-in file because it is missing

After all, we want to be able to boot with /etc empty one day...

(cherry picked from commit 9f43a07f10)

src/shared/conf-parser.c

@@ -336,8 +336,8 @@ int config_parse(const char *unit,
         if (!f) {
                 f = ours = fopen(filename, "re");
                 if (!f) {
-                        log_error("Failed to open configuration file '%s': %m", filename);
-                        return -errno;
+                        log_full(errno == ENOENT ? LOG_DEBUG : LOG_ERR, "Failed to open configuration file '%s': %m", filename);
+                        return errno == ENOENT ? 0 : -errno;
                 }
         }