Bundle validation: verify with charms.

Validate the bundle with respect to charms in the store.
Does it make sense to do change this into: 
func (h *handler) bundleCharms(ids []string) (map[string]charm.Charm, error) {
  return bundleCharmsSequentialGet(ids) {
} 
and renaming current func into:
func (h *handler) bundleCharmsSequentialGet(ids []string) (map[string]charm.Charm, error) {
so that:
a) it is clear that it is sequential
b) it is easy to add concurrent check