baloosearchruner: Emit DBus error when baloo is disabled

Alexander Lohnau requested to merge work/baloo_runner_error into master

Otherwise we exit and KRunner runner into a long timeout.

By making this a dedicated DBus error KRunner can also know, that this runner can not be started. Consequently making new attempts to query this runner can be prevented.

Because the runner is DBus-Activated, the result of exiting in the Actions method or directly in the main method is practically the same.

CCBUG: 445342

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