Add unit tests to SQL exception mappers in dropwizard-jdbi

Some unit tests for JDBI exception mappers. 
They check that we log correct things when an exception from JDBI ocurrs.
Instead of turning this into an instance variable you could make `LOGGER` non-final and create a package-protected static method `setLogger` which is annotated with `@VisibleForTesting` (from Guava).

That way normal users won't see the "hack" necessary for testing the ExceptionMapper.