Bundle validation: verify with charms.

Validate the bundle with respect to charms in the store.
This doesn't seem quite right. If URL resolving fails, we can be left
with empty slots here.

I'd suggest:
    urls := make([]*charm.Reference, 0, len(ids))

and then append to urls after parsing each one.

It would be good to have some better test coverage in this area. Perhaps
a unit test of bundleCharms itself?