locking: Don't fail in error handling if lock isn't owned

In case an exception was thrown while acquiring the lock, not necessarily all
owned locks are also really acquired. Before this change, an exception could be
masked by another exception thrown here. There is no good clean-up strategy
when acquiring a lock fails with an exception in either case.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>