exec-util: add missing logging call

This function logs about all errors, but one case was forgotten. Fix
that.

src/basic/exec-util.c

@@ -87,7 +87,7 @@ static int do_execute(
 
         r = conf_files_list_strv(&paths, NULL, NULL, CONF_FILES_EXECUTABLE|CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED, (const char* const*) directories);
         if (r < 0)
-                return r;
+                return log_error_errno(r, "Failed to enumerate executables: %m");
 
         if (!callbacks) {
                 pids = hashmap_new(NULL);