ProblemManager: Replica Set not degraded if slaves unreachable

[problame]

there should be some kind of error if a slave in a replica set is unreachable.
(i.e. when a Slave's ObservationError != NULL)

this should be visualized in the GUI (maybe this already happens, currently, the right problem is not generated)

• list of replica sets: the affected replica set should be in the list of replica sets with problems
• replica set view: the "Replica Set Overview" section rows should highlight a slave with problems with red background or similar