[sfntedit] Ignore missing tables and continue

[twardoch]

@readroberts Currently, when I use sfntedit to extract or delete a series of tables, and specify some tables that don't exist in the font (e.g. I say sfntedit -d GPOS,GSUB font.ttf and the font does not have GSUB but only GPOS), sfntedit issues an error sfntedit [FATAL]: table missing (GSUB) and does not perform the operation at all. It would be useful to have an option -i (ignore errors) that would ignore these situations and still perform the operations on the other tables which do exist. I don't know enough C to patch sfntedit myself but I think it should be a relatively minor but useful thing.