Make LibraryImporter a little more robust

...alternative title, "Let me import UnfoldingMaps because I want to use it in a demonstration tomorrow."
Partially addresses [#85](https://github.com/jdf/Processing.py-Bugs/issues/85) and [#86](https://github.com/jdf/Processing.py-Bugs/issues/86), by allowing LibraryImporter to skip over things it knows will be problematic, and to recover when imports don't work. It's probably a little too permissive, but I want to err on the side of libraries *mostly* working for now.
Under what conditions was this happening in the first place?