Add early return to Redis watcher in case of MGET error to avoid panicking

[Oberonus]

Which problem is this PR solving?

In case of MGET error current implementation still expects resulting values array to have the same length as keys array, tries to iterave over them and panicks with:

runtime error: index out of range [0]
github.com/beatlabs/harvester/monitor/redis.(*Watcher).
      getValues(0xc00011dad0, 0x1516268, 0xc000042050, 
      /go/sonar/vendor/github.com/beatlabs/harvester/monitor/redis/watcher.go:75
github.com/beatlabs/harvester/monitor/redis.(*Watcher).
      monitor(0xc00011dad0, 0x1516268, 0xc000042050, 0xc0001311a0) 
      /go/sonar/vendor/github.com/beatlabs/harvester/monitor/redis/watcher.go:62

Short description of the changes

Add an early return in case of error.