According to API:

OpenDesignFile opens an existing design file after closing the currently open design file.
If an error is raised during this method, the original design file is left open.

But no error is actually raised if opened file is not valid for example renamed txt file as dgn.

VBA example not valid as errortrap does nothing:

Example

    Sub main()
        Dim fileName As String

        fileName = "D:\Bentley\Workspace\Projects\Examples\General\dgn\FlowTags.dgn"

        On Error GoTo errorTrap
        OpenDesignFile fileName, False, msdV7ActionWorkmode
        On Error GoTo 0

        Exit Sub

        errorTrap:
            MsgBox Err.Description
            Resume Next
    End Sub