Provide full exception details when karaf doesn't startup

I got a a message like so when starting Karaf today:

-bash-3.2$ ./bin/karaf
Karaf can't startup, make sure the log file can be accessed and written by the user starting Karaf : /export/home/hudson/janstey/apache-karaf-2.3.3/system/org/apache/felix/org.apache.felix.framework/4.0.3/org.apache.felix.framework-4.0.3.jar

It is not really clear from the message what the user has to do. This can be improved.