Clarify javadoc of ExecutionPlanBuilder.execute()

Currently the https://github.com/apache/jackrabbit-filevault/blob/f785fcb24d4cbd01c734e9273310a925c29ae15b/vault-core/src/main/java/org/apache/jackrabbit/vault/packaging/registry/impl/ExecutionPlanBuilderImpl.java#L331 does not throw an exception in case a task fails. This is due to the fact that all errors are caught in https://github.com/apache/jackrabbit-filevault/blob/b044c03e4440c935fb1b49a41771551307eb3a3a/vault-core/src/main/java/org/apache/jackrabbit/vault/packaging/registry/impl/PackageTaskImpl.java#L91 and just collected.

This requires clarification in https://github.com/apache/jackrabbit-filevault/blob/b044c03e4440c935fb1b49a41771551307eb3a3a/vault-core/src/main/java/org/apache/jackrabbit/vault/packaging/registry/ExecutionPlanBuilder.java#L113 as it is currently states there "throws PackageException if a package operation fails."