Don't show incomplete experiments in user interface

[bfirsh]

When experiments are created, the metadata is created first and the data is uploaded second. This will cause failure when you try to check it out, because data doesn't exist yet.

Checkpoints don't suffer from this issue -- the metadata is saved after data is uploaded.

I tried fixing this but tests fail so it doesn't look trivial. I think it's an issue where a checkpoint gets created before metadata is saved, it looks for the metadata file and breaks. Maybe this needs to be queued more intelligently, or maybe we need a flag in the metadata to hide it in the UI.

As mentioned in #408 comments