Improvements to error handling/reporting

Library parts of Slinky no longer write to console, but instead through
exceptions, which are caught by application code (server, builder). Errors
can be batched using SlinkyError.batch_errors.
Slinky will now report errors to the browser if enabled

Error injection is support for HTML, JS and CSS products

add `enable_browser_errors: true` to config to use.
Merge pull request #6 from mwylde/feature_better_errors

Better errors