[FIX] account: verify that account exists before automatic reconcile
[FIX] account: verify that account exists before automatic reconcile

When using the fast_counterpart creation method to automatically
reconcile bank statement lines, there was no verification that the
account id was not set or that the statement was already linked to journal
entries. It was causing a traceback in some conditions when the
demo data were loaded because the fast_counterpart_creation was called
without the verification, using an XML function.

This commmit introduce the necessary verifcation.
Thanks @qdp-odoo for the review and some docstrings/comments

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