Refactor error logging and exception handling

Currently, there seem to be two patterns for handling exceptions: 1) catch and log a warning, 2) propagate the exception up to the main method.

The goal of this task is to:

1. Decide which exceptions are safe to proceed with. These should be logged and not propagated.
2. Decide which exceptions are fatal. These should be logged and propagated (possibly wrapped in a custom exception type) and should cause SmartSprites to terminate.
3. Remove throws classes with {{IOException}}s from the call hierarchy, replace with the custom exception type introduced in point 2.
4. Extend unit test coverage of fatal errors.
5. Throw a fatal exception in org.carrot2.labs.smartsprites.resource.FileSystemResourceHandler.getResourcePath(String, String) when there is an absolute path to resolve, but document root is not specified.