JBPM-3913: Fixed WorkflowRuntimeException so that process variables are returned.
JBPM-3913: Fixes to Exception handling decorators
JBPM-3913: Add correct xml dumping of processes that use <error> elements
JBPM-3913: tests for error signals/work item handler implementations
Clean up of jbpm-bpmn2 tests (added redumping and weeded out unused/unnecessary base class tests)
JBPM-3913: Exception handling and improved work item documentation

- Added an exception handling chapter (documentation) and examples
- Rewrote the "Domain-Specific Processes" chapter
- Added (exception handling) examples that are referenced in the documentation
- Moved certain sections to the Designer and Eclipse plugin chapters
JBPM-3785: disallowing use of Compensation Boundary Events (check at compile time) until we support them