Expose RepositoryRuntimeException from the kernel module instead of RepositoryException

Far from perfect, but getting closer. At least all the resource-like models are sanitized.

https://www.pivotaltracker.com/story/show/78860776
Apparently the "showStackTrace" boolean was carried over from WildCardExceptionMapper... where it is toggled in test code. The boolean in RepositoryExceptionMapper and RepositoryRuntimeExceptionMapper is never used.
Dropping them both seems to make the most sense, either here or in a separate commit.