release: elide 'remote end hung up' error on first release.
release: elide 'remote end hung up' error on first release.

* build-aux/release.mk (check-in-release-branch): Elide the
misleading 'remote end hung up' error when failing to pull the
not-yet-created release branch updates into the local tree.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot