[keycloak-dev] Handling exceptions thrown from user storage providers

Currently there isn't much handling of exceptions which may be thrown from User Storage SPI methods. For example: - when `UserStorageProvider.preRemove` throws an exception it's not possible to remove a group/role or realm in which the offending provider is registered - when `UserLookupProvider.getUserByUsername` throws an exception this blocks all lower priority providers from looking up users - exception from `UserStorageProviderFactory.create` blocks all providers from lookup, credential validation/updates, etc. I can't decide whether this "fail fast" behavior is bug or feature. On one hand it lets the developer know there's a problem with the provider. On the other hand it destabilizes the other providers in the realm. WDYT? Tomas