DocumentApp.PositionedImage#getBlob can throw an exception

[ggrossetie]

I don't know exactly why and this is undocumented but I can reproduce this issue on a Google Document.
For reference, here's the documentation for DocumentApp.PositionedImage#getBlob: https://developers.google.com/apps-script/reference/document/positioned-image

As a result, we should add a try/catch around:

https://github.com/Mogztter/asciidoc-googledocs-addon/blob/e629db563e5bba80885a829ca840590549378320/app/Code.gs#L322-L324