A new feature as of BI Platform 3.6 is template error pages. There used to be no easy way to configure an error page resulting from an action sequence execution from the ViewAction servlet. We used to direct customers to create their own (Java code) MessageFormatter class if they want to customize the way the error appears to end users. The error used to be ugly, plain and quite uniformative. The uninformative messages "Action Sequence Failed" is affectionately known as the SBC, or "something broke cap'n" error. As of 3.6 you no longer are stuck with the SBC error when something goes wrong with your action sequence!
Extensive changes took place in the internals of the platform to render lots of detail about xaction failures. These details are now exposed in the templated error page mentioned above, but also in the pentaho.log file (as well as your appserver's log file, e.g. catalina.out).