[Bug] Bad image creation preview kills entire workflow

[bendhouseart]

Determined that one bad subject image, --pics option used with MiDeface can spoil the workflow by throwing the following exception:

FileNotFoundError: No such file or directory '/AMHV_defaced/petdeface_wf/single_subject_nbdamhv04_wf/t1w_sub-nbdamhv04_wf/deface_t1w_sub-nbdamhv04/sub-nbdamhv04.face-before.png' for output 'out_before_pic' of a Mideface interface

Disabling pics allows this to run to completion, but it would be ideal that it could better handle this very possible eventuality.