vault/assets: Fix cooking task creation when there is a pending one

When requesting a vault cooking creation but there is already a pending one
for the same SWHID, ensure task creation modal is displayed as previously an
error was erroneoulsy reported in the Web UI.

Under the hood, no new cooking task will be created but the pending one will
be displayed in the vault UI. Nevertheless the email possibly submitted by the
user in the modal will be added to the list of emails to notify by the vault
backend.

Related to T4698