Added a new package for exception classes.

Currently, exception classes defined by this library are located in the
top-level package. If sub packages throw exceptions of these types, this may
cause cyclic dependencies. To avoid this, all exceptions classes are moved into
a separate package.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1554633 13f79535-47bb-0310-9956-ffa450edef68