Clarify error message when trying to add Extension to asset with no owner

[lossyrob]

Just talked with a user who was using the ProjectionExtension to add projection information to assets. They had the add_if_missing=True flag set. The assets were being created before they were being attached to the Item, so the error here was thrown:

Can only add schema URIs to Assets with an owner.

The user wasn't clear on what the error message meant. There was some explination about how Assets and Items can hold projection extension information, but on Items can hold the Schema URI's, which act as the IDs for the extension that must be declared. Things got cleared up, but I think that the error message was a bit confusing and could be cleared up to point a user who's newer to usage of extensions to the correct fix.