burnin: fix removal errors hiding real errors

A long-standing bug in burnin makes errors during the removal phase
(e.g. because an import has failed, or because the initial creation has
failed) hide the original error.

This patch suppresses removal errors if we are already in ‘has_err’
mode, and otherwise it displays them normally.

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