[NEXUS-6004] replace CompositeException usage with helper to attach suppressed exceptions 

https://issues.sonatype.org/browse/NEXUS-6004

Java7 API provides the ability to attach _suppressed_ exceptions which is essentially what CompositeException is attempting to do.

NOTE: commented change for helper is in goodies, but its no Java7 yet so it can't live there:

* https://github.com/sonatype/sisu-goodies/commit/d87688878f282bbc4be3419e72d1ea491984d251

For now I've put these same helpers onto CompositeException, and replaced all usage of that class with the helper.