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.
I'm uncomfortable with this. If an import fails, it should fail right away, rather than when the code refers to a class it tried to import.

What does it mean for a library to optionally require another library? What is it doing in Java?