[FIX] base: Fix wrong wording for error when archiving a contact with a user still linked to it

Description of the issue/feature this PR addresses and current behavior before PR:
There is a discrepancy in the wording of the error message provided. The error reads that an 'internal user' is linked but really its a 'portal user.'.

Desired behavior after PR is merged:
The wording of the error is clearer, and it now says 'portal user' instead of 'internal user'.

OPW: 2498139

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closes odoo/odoo#69550

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>