add IndexBuildAborted to list of acceptable errors in fsm_workloads/create_index_background_unique.js

Starting In SERVER-43612, the create_background_index_unique.js concurrency test workload may get a IndexBuildAborted error from the createIndexes command if there is an existing index build currently being torn down. Previously, we would have gotten a IndexBuildAlreadyInProgress error. This ticket proposes to make IndexBuildAborted an acceptable error for this test workload (similar to what's being done for IndexBuildAlreadyInProgress)