lib: Add error prefixing for sysroot load and repo open

Noticed this while writing tests for a core `ostree_sysroot_load()`
entrypoint.  And decided to do the same for `ostree_repo_open()`,
and while there also noted we had a duplicate error prefixing
for the open (more recently `glnx_opendirat()` automatically
prefixes with the path).

src/libostree/ostree-sysroot.c

@@ -1003,6 +1003,7 @@ ostree_sysroot_load_if_changed (OstreeSysroot  *self,
                                 GCancellable   *cancellable,
                                 GError        **error)
 {
+  GLNX_AUTO_PREFIX_ERROR ("loading sysroot", error);
   if (!ostree_sysroot_initialize (self, error))
     return FALSE;