This package is ancient. Recently we have been moving to more
self-contained top-level packages under Gerrit, so follow that pattern
and break out a separate build rule.
Use the name "exceptions" instead of "errors" to avoid giving the
impression that this contains only java.lang.Errors.
Change-Id: I727b73624d6f57bcc55d9842f7a36e091a175cbe
9dd73c8a71