No warning for circular dependencies

[stakach]

I came across an issue today where we had a circular dependency https://github.com/PlaceOS/driver/issues/105

however an earlier version of this shard didn't have the circular dependency so was being installed without warning I couldn't work out why shards wasn't installing the latest version, I bumped my shards yml to force its hand and then got the error about circular dependencies.

It would be nice if shards would hint as to why it didn't select the latest version. Like Installing placeos-driver (0.4.7) - There is a circular dependency between placeos-driver and placeos-core with 0.5.0

unsure how difficult this would be