Mapnik CSV error handling [WIP]

[who8mycakes]

Next steps:

• [ ] Properly handle and shorten the error by removing - this is required for reading geometry data from the error message, and/or truncate all error messages to 256 characters.
• [ ] Make sure all datasource creation steps in mapnik-omnivore are wrapped in a try/catch statement.
• [ ] Add invalid and valid case unit tests