"Does anyone know how to trap the error that occurs when a user is not logged onto the network and attempts open the front end?"

Try this to check for the drive letter:

Function CheckDrive(whichone) As Boolean
    Dim z
    On Error GoTo err_checkdrive
    z = Dir(whichone, vbDirectory)
    CheckDrive = True
    Exit Function
    CheckDrive = False
    Resume exit_checkdrive
End Function

Use it like this: CheckDrive("Q:") returns true if it exists, false if it does not. It should also work for UNC drives like CheckDrive("\\Shaz"). If this function returns false, then alert the user.