`salishsea prepare` cleanup fails when restart file is not found

[douglatornell]

Original report by Doug Latornell (Bitbucket: douglatornell, GitHub: douglatornell).

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Nancy reported that salishsea prepare did not delete the partially populated temporary run directory when it stopped due to the specified restart file not being found.

[douglatornell]

Need to confirm that this issue can be reproduced.