In the case of non-existent bridge interface, the command, called by _get_raw_data_fdb, called by show_fdb:

sudo bridge --json fdb show br {bridge}

will return exit code -1. The source code for iproute2 'show_fdb' reveals that this is the only condition to return -1 for that command, consequently we can unambiguously raise the op-mode error 'UnconfiguredSubsystem' in this case.