tratar erro ao att gridview

O seguinte erro dá quando tiro o focus da linha que estou inserindo na base.   queria saber onde tratar e informar ao usuario que ja existe um cadastro com esse nome de logon :)   no aguardo.     ---------------------------
DataGridView Default Error Dialog
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The following exception occurred in the DataGridView:   System.Data.ConstraintException: Column 'logon' is constrained to be unique.  Value 'paulovitor' is already present.    at System.Data.UniqueConstraint.CheckConstraint(DataRow row, DataRowAction action)    at System.Data.DataTable.RaiseRowChanging(DataRowChangeEventArgs args, DataRow eRow, DataRowAction eAction, Boolean fireEvent)    at System.Data.DataTable.SetNewRecordWorker(DataRow row, Int32 proposedRecord, DataRowAction action, Boolean isInMerge, Boolean suppressEnsurePropertyChanged, Int32 position, Boolean fireEvent, Exception& deferredException)    at System.Data.DataTable.InsertRow(DataRow row, Int64 proposedID, Int32 pos, Boolean fireEvent)    at System.Data.DataView.FinishAddNew(Boolean success)    at System.Data.DataRowView.EndEdit()    at System.Windows.Forms.CurrencyManager.EndCurrentEdit()    at System.Windows.Forms.DataGridView.DataGridViewDataConnection.OnRowValidating(DataGridViewCellCancelEventArgs e)   To replace this default dialog please handle the DataError event.
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OK  
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