transaction: remove DBusError from function signature of transaction_merge_jobs()
transaction: rewrite function signatures of transaction_verify_order*() so as to omit DBusError
transaction: transaction_is_destructive() returns standard errno, not DBusError
transaction: overhaul transaction_add_job_and_dependencies()
manager: use updated signature for transaction_add_job_and_dependencies()
transaction, manager: transaction_activate() without DBusError setting