REVIEW: [NEXUS-6771] if an error occurs during Jetty startup, shutdown the container

https://issues.sonatype.org/browse/NEXUS-6771
http://bamboo.s/browse/NX-OSSF227-2 :white_check_mark: 

Note: this behaviour can be disabled by setting the 'org.sonatype.nexus.bootstrap.ShutdownHelper.ignore' system property to true.

PS. I've also added some minor improvements to the general boot process to this PR - feel free to veto any you don't feel add to the current setup (such as logging activation of individual bundles, which imho gives better feedback of where we are in the boot process, but could also be seen as log spam.)