return error when garbageThreshold is not a valid float number
return error when garbageThreshold is not a valid float number

Signed-off-by: Lei Liu <734bd247007e81fc996b93142682515e47b2245c@horizon.ai>

chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs