import: two minor debugability improvements (#42081)

TEST-13-NSPAWN.machined occasionally fails when importing, and it's hard
to debug, so try to make it better. eg:


https://github.com/systemd/systemd/actions/runs/25800895182/job/75790334230?pr=42071

src/import/import-common.c

@@ -392,13 +392,14 @@ int import_remove_tree(const char *path, int *userns_fd, ImportFlags flags) {
         assert(path);
         assert(userns_fd);
 
-        r = import_make_foreign_userns(userns_fd);
-        if (r < 0)
-                return r;
-
         /* Try the userns dance first, to remove foreign UID range owned trees */
-        if (FLAGS_SET(flags, IMPORT_FOREIGN_UID))
+        if (FLAGS_SET(flags, IMPORT_FOREIGN_UID)) {
+                r = import_make_foreign_userns(userns_fd);
+                if (r < 0)
+                        return r;
+
                 (void) remove_tree_foreign(path, *userns_fd);
+        }
 
         r = rm_rf(path, REMOVE_ROOT|REMOVE_PHYSICAL|REMOVE_SUBVOLUME|REMOVE_MISSING_OK|REMOVE_CHMOD);
         if (r < 0)

src/import/import-tar.c

@@ -368,6 +368,17 @@ static int tar_import_process(TarImport *i) {
         r = decompressor_push(i->compress, i->buffer, i->buffer_size, tar_import_write, i);
         if (r < 0) {
                 log_error_errno(r, "Failed to decode and write: %m");
+
+                /* Try to check the actual exit code from the child process, to make debugging easier */
+                if (r == -EPIPE && pidref_is_set(&i->tar_pid)) {
+                        int q = pidref_wait_for_terminate_and_check("tar", &i->tar_pid, WAIT_LOG);
+                        pidref_done(&i->tar_pid);
+                        if (q < 0)
+                                r = q;
+                        else if (q != EXIT_SUCCESS)
+                                r = -EPROTO;
+                }
+
                 goto finish;
         }