only use plugin after truthiness check

only use plugin after truthiness check

Commit 1a87db9e566599d02e62d9daf4a248ef54e00469 introduced
a usage of plugin before the truthiness check for plugin.

At the moment it might not be possible to trigger this anymore,
because of the guest inclusion rework that happened later on.
But to make tasks for inexistent guest IDs visibly fail again,
this check will be necessary. Also, to get the error message in
the mail, it needs to fail inside the eval block.

Thus, keep the check in the eval block and move the block of code
using the plugin to below the check.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>