Exception handling

[edemaine]

As discussed here, if a CoffeeScript/JavaScript tile definition throws an exception, it should probably render as a special "ERROR" SVG (similar to "UNRECOGNIZED", but maybe a different geometry). Of course, it should also report the error on the command line (and maybe also add it as a title attribute so hover can reveal which error happens where?). Also, it should not continue by looking up in the next Mapping. On the other hand, returning null/undefined should encourage looking up in the next Mapping. I think most of this is already the current behavior, but we should document it.