Fix IPluginInBridgeDomainConfigService API

IPluginInBridgeDomainConfigService protomoted Throwable into a
checked-exception status. This also includes Errors, which is not a good
thing, as Errors are not something we can handle gracefully.

Introduce a dedicated exception which implementations can use to report
instantiation problems.

Change-Id: Ib16d792acfcf77c68b92b35526ed7471e2172cbe
Signed-off-by: Robert Varga <rovarga@cisco.com>