refs.c: update ref_transaction_delete to check for error and return status

refs.c: update ref_transaction_delete to check for error and return status

Change ref_transaction_delete() to do basic error checking and return
non-zero on error. Update all callers to check the return for
ref_transaction_delete(). There are currently no conditions in _delete that
will return error but there will be in the future. Add an err argument that
will be updated on failure.

Signed-off-by: Ronnie Sahlberg <redacted>
Reviewed-by: Michael Haggerty <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>